diff --git a/.github/workflows/build_and_test.yml b/.github/workflows/build_and_test.yml
index 1f4fb64e6b07d9..9a0e0cbb8e95bb 100644
--- a/.github/workflows/build_and_test.yml
+++ b/.github/workflows/build_and_test.yml
@@ -502,6 +502,7 @@ jobs:
 
         export NEXT_TEST_MODE=start
         export NEXT_TEST_WASM=true
+        export IS_WEBPACK_TEST=1
         node run-tests.js \
           test/production/pages-dir/production/test/index.test.ts \
           test/e2e/streaming-ssr/index.test.ts
@@ -711,7 +712,7 @@ jobs:
 
     secrets: inherit
 
-  test-dev:
+  test-dev: # TODO: rename to include webpack
     name: test dev
     needs: ['optimize-ci', 'changes', 'build-native', 'build-next']
     if: ${{ needs.optimize-ci.outputs.skip == 'false' && needs.changes.outputs.docs-only == 'false' }}
@@ -728,6 +729,7 @@ jobs:
     uses: ./.github/workflows/build_reusable.yml
     with:
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_MODE=dev
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
@@ -752,8 +754,10 @@ jobs:
 
     uses: ./.github/workflows/build_reusable.yml
     with:
+      # Should this be using turbopack? a variation?
       afterBuild: |
         export NEXT_TEST_MODE=dev
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           test/e2e/app-dir/app/index.test.ts \
@@ -780,6 +784,7 @@ jobs:
     with:
       nodeVersion: 20.9.0
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         node run-tests.js \
           --concurrency 4 \
           test/production/pages-dir/production/test/index.test.ts \
@@ -808,6 +813,7 @@ jobs:
     with:
       afterBuild: |
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js --type production \
           test/e2e/app-dir/app/index.test.ts \
@@ -836,6 +842,7 @@ jobs:
     uses: ./.github/workflows/build_reusable.yml
     with:
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_MODE=start
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
@@ -875,6 +882,7 @@ jobs:
     with:
       nodeVersion: 20.9.0
       afterBuild: |
+        export IS_WEBPACK_TEST=1
         export NEXT_TEST_REACT_VERSION="${{ matrix.react }}"
 
         node run-tests.js \
@@ -896,6 +904,7 @@ jobs:
 
         # these all run without concurrency because they're heavier
         export TEST_CONCURRENCY=1
+        export IS_WEBPACK_TEST=1
 
         BROWSER_NAME=firefox node run-tests.js \
           test/production/pages-dir/production/test/index.test.ts
@@ -923,6 +932,7 @@ jobs:
       afterBuild: |
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -945,6 +955,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
         export NEXT_TEST_MODE=dev
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -968,6 +979,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/ppr-tests-manifest.json"
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -990,6 +1002,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_PPR=true # for compatibility with the existing tests
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -1014,6 +1027,7 @@ jobs:
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=dev
         export __NEXT_EXPERIMENTAL_ISOLATED_DEV_BUILD=true
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
@@ -1038,6 +1052,7 @@ jobs:
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=start
+        export IS_WEBPACK_TEST=1
 
         node run-tests.js \
           --timings \
diff --git a/.github/workflows/pull_request_stats.yml b/.github/workflows/pull_request_stats.yml
index 3b3afeee5da3b3..9fe1fc91701bc8 100644
--- a/.github/workflows/pull_request_stats.yml
+++ b/.github/workflows/pull_request_stats.yml
@@ -60,3 +60,7 @@ jobs:
 
       - uses: ./.github/actions/next-stats-action
         if: ${{ steps.docs-change.outputs.DOCS_CHANGE == 'nope' }}
+        env:
+          # This uses the webpack bundle analyzer and for consistent results we need to use webpack.
+          # Once there is an equivalent analyzer for turbopack, we can remove this.
+          IS_WEBPACK_TEST: 1
diff --git a/.github/workflows/test_e2e_deploy_release.yml b/.github/workflows/test_e2e_deploy_release.yml
index 631b0a2b43f462..9c0d33eeefde9b 100644
--- a/.github/workflows/test_e2e_deploy_release.yml
+++ b/.github/workflows/test_e2e_deploy_release.yml
@@ -78,7 +78,15 @@ jobs:
       matrix:
         group: [1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, 8/8]
     with:
-      afterBuild: npm i -g vercel@latest && NEXT_E2E_TEST_TIMEOUT=240000 NEXT_TEST_MODE=deploy NEXT_EXTERNAL_TESTS_FILTERS="test/deploy-tests-manifest.json" NEXT_TEST_VERSION="${{ github.event.inputs.nextVersion || 'canary' }}" VERCEL_CLI_VERSION="${{ github.event.inputs.vercelCliVersion || 'vercel@latest' }}" node run-tests.js --timings -g ${{ matrix.group }} -c 2 --type e2e
+      afterBuild: |
+        npm i -g vercel@latest && \
+        NEXT_E2E_TEST_TIMEOUT=240000 \
+        NEXT_TEST_MODE=deploy \
+        IS_WEBPACK_TEST=1 \
+        NEXT_EXTERNAL_TESTS_FILTERS="test/deploy-tests-manifest.json" \
+        NEXT_TEST_VERSION="${{ github.event.inputs.nextVersion || 'canary' }}" \
+        VERCEL_CLI_VERSION="${{ github.event.inputs.vercelCliVersion || 'vercel@latest' }}" \
+        node run-tests.js --timings -g ${{ matrix.group }} -c 2 --type e2e
       skipNativeBuild: 'yes'
       skipNativeInstall: 'no'
       stepName: 'test-deploy-${{ matrix.group }}'
diff --git a/bench/heavy-npm-deps/package.json b/bench/heavy-npm-deps/package.json
index 2428e307218e85..8bd442a10f8a5e 100644
--- a/bench/heavy-npm-deps/package.json
+++ b/bench/heavy-npm-deps/package.json
@@ -4,9 +4,9 @@
   "private": true,
   "scripts": {
     "dev-turbopack": "next dev --turbopack",
-    "dev-webpack": "next dev",
+    "dev-webpack": "next dev --webpack",
     "build-turbopack": "next build --turbopack",
-    "build-webpack": "next build",
+    "build-webpack": "next build --webpack",
     "start-turbopack": "next start",
     "start-webpack": "next start",
     "build-application": "next build",
diff --git a/bench/module-cost/package.json b/bench/module-cost/package.json
index eb6e8a7478d92a..96814e0e6e8759 100644
--- a/bench/module-cost/package.json
+++ b/bench/module-cost/package.json
@@ -3,10 +3,10 @@
   "scripts": {
     "prepare-bench": "node scripts/prepare-bench.mjs",
     "benchmark": "node scripts/benchmark-runner.mjs",
-    "dev-webpack": "next dev",
-    "dev-turbopack": "next dev --turbo",
-    "build-webpack": "next build",
-    "build-turbopack": "next build --turbo",
+    "dev-webpack": "next dev --webpack",
+    "dev-turbopack": "next dev --turbopack",
+    "build-webpack": "next build --webpack",
+    "build-turbopack": "next build --turbopack",
     "start": "next start"
   },
   "devDependencies": {
diff --git a/package.json b/package.json
index bd4eb8ca637c44..b36ba0bcd23c32 100644
--- a/package.json
+++ b/package.json
@@ -17,29 +17,49 @@
     "ci:publish": "tsx ./scripts/release/publish-npm.ts",
     "test-types": "tsc",
     "test-unit": "jest test/unit/ packages/next/ packages/font",
-    "test-dev": "cross-env NEXT_TEST_MODE=dev pnpm testheadless",
-    "test-dev-experimental": "cross-env NEXT_TEST_MODE=dev pnpm run with-experimental pnpm testheadless",
-    "test-dev-rspack": "pnpm run with-rspack pnpm run test-dev",
-    "test-dev-experimental-rspack": "pnpm run with-rspack pnpm run test-dev-experimental",
-    "test-dev-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testheadless",
-    "test-dev-experimental-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm run with-experimental pnpm testheadless",
-    "test-start": "cross-env NEXT_TEST_MODE=start pnpm testheadless",
-    "test-start-experimental": "cross-env NEXT_TEST_MODE=start pnpm run with-experimental pnpm testheadless",
-    "test-start-rspack": "pnpm run with-rspack pnpm run test-start",
-    "test-start-experimental-rspack": "pnpm run with-rspack pnpm run test-start-experimental",
-    "test-start-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testheadless",
-    "test-start-experimental-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm run with-experimental pnpm testheadless",
-    "test-deploy": "cross-env NEXT_TEST_MODE=deploy pnpm testheadless",
-    "testonly-dev": "cross-env NEXT_TEST_MODE=dev pnpm testonly",
-    "testonly-dev-rspack": "pnpm run with-rspack pnpm run testonly-dev",
-    "testonly-dev-turbo": "cross-env NEXT_TEST_MODE=dev IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testonly",
-    "testonly-start": "cross-env NEXT_TEST_MODE=start pnpm testonly",
-    "testonly-start-rspack": "pnpm run with-rspack pnpm run testonly-start",
-    "testonly-start-turbo": "cross-env NEXT_TEST_MODE=start IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly",
-    "testonly-deploy": "cross-env NEXT_TEST_MODE=deploy pnpm testonly",
-    "test": "pnpm testheadless",
-    "test-rspack": "pnpm run with-rspack pnpm run test",
-    "test-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 TURBOPACK_BUILD=1 pnpm testheadless",
+    "test-dev-inner": "cross-env NEXT_TEST_MODE=dev pnpm testheadless",
+    "test-dev": "pnpm run test-dev-webpack",
+    "test-dev-webpack": "pnpm run with-webpack pnpm test-dev-inner",
+    "test-dev-experimental-inner": "pnpm run with-experimental pnpm test-dev-inner",
+    "test-dev-experimental": "pnpm run test-dev-experimental-webpack",
+    "test-dev-experimental-webpack": "pnpm run with-webpack pnpm test-dev-experimental-inner",
+    "test-dev-rspack": "pnpm run with-rspack pnpm run test-dev-inner",
+    "test-dev-experimental-rspack": "pnpm run with-rspack pnpm run test-dev-experimental-inner",
+    "test-dev-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm test-dev-inner",
+    "test-dev-experimental-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm run with-experimental pnpm test-dev-inner",
+    "test-start-inner": "cross-env NEXT_TEST_MODE=start pnpm testheadless",
+    "test-start": "pnpm run test-start-webpack",
+    "test-start-webpack": "pnpm run with-webpack pnpm run test-start-inner",
+    "test-start-experimental-inner": "pnpm run with-experimental pnpm test-start-inner",
+    "test-start-experimental": "pnpm run test-start-experimental-webpack",
+    "test-start-experimental-webpack": "pnpm run with-webpack pnpm run test-start-experimental-inner",
+    "test-start-rspack": "pnpm run with-rspack pnpm run test-start-inner",
+    "test-start-experimental-rspack": "pnpm run with-rspack pnpm run test-start-experimental-inner",
+    "test-start-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm test-start-inner",
+    "test-start-experimental-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm run with-experimental pnpm test-start-experimental-inner",
+    "test-deploy-inner": "cross-env NEXT_TEST_MODE=deploy pnpm testheadless",
+    "test-deploy": "pnpm run test-deploy-webpack",
+    "test-deploy-webpack": "pnpm run with-webpack pnpm test-deploy-inner",
+    "test-deploy-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm test-deploy-inner",
+    "testonly-dev-inner": "cross-env NEXT_TEST_MODE=dev pnpm testonly",
+    "testonly-dev": "pnpm run testonly-dev-webpack",
+    "testonly-dev-webpack": "pnpm run with-webpack pnpm run testonly-dev-inner",
+    "testonly-dev-rspack": "pnpm run with-rspack pnpm run testonly-dev-inner",
+    "testonly-dev-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 pnpm testonly-dev-inner",
+    "testonly-start-inner": "cross-env NEXT_TEST_MODE=start pnpm testonly",
+    "testonly-start": "pnpm run testonly-start-webpack",
+    "testonly-start-webpack": "pnpm run with-webpack pnpm run testonly-start-inner",
+    "testonly-start-rspack": "pnpm run with-rspack pnpm run testonly-start-inner",
+    "testonly-start-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly-start-inner",
+    "testonly-deploy-inner": "cross-env NEXT_TEST_MODE=deploy pnpm testonly",
+    "testonly-deploy": "pnpm run testonly-deploy-webpack",
+    "testonly-deploy-webpack": "pnpm run with-webpack pnpm run testonly-deploy-inner",
+    "testonly-deploy-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_BUILD=1 pnpm testonly-deploy-inner",
+    "test-inner": "pnpm testheadless",
+    "test": "pnpm test-webpack",
+    "test-webpack": "pnpm run with-webpack pnpm run test-inner",
+    "test-rspack": "pnpm run with-rspack pnpm run test-inner",
+    "test-turbo": "cross-env IS_TURBOPACK_TEST=1 TURBOPACK_DEV=1 TURBOPACK_BUILD=1 pnpm test-inner",
     "testonly": "jest --runInBand",
     "testheadless": "cross-env HEADLESS=true pnpm testonly",
     "genstats": "cross-env LOCAL_STATS=true node .github/actions/next-stats-action/src/index.js",
@@ -89,6 +109,7 @@
     "build-storybook": "turbo run build-storybook",
     "test-storybook": "turbo run test-storybook",
     "with-rspack": "cross-env NEXT_RSPACK=1 NEXT_TEST_USE_RSPACK=1",
+    "with-webpack": "cross-env IS_WEBPACK_TEST=1",
     "with-experimental": "cross-env __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true __NEXT_EXPERIMENTAL_PPR=true"
   },
   "devDependencies": {
diff --git a/packages/next-rspack/index.js b/packages/next-rspack/index.js
index e0ce02d637d847..59bf459282dd60 100644
--- a/packages/next-rspack/index.js
+++ b/packages/next-rspack/index.js
@@ -1,5 +1,15 @@
 module.exports = function withRspack(config) {
   process.env.NEXT_RSPACK = 'true'
   process.env.RSPACK_CONFIG_VALIDATE = 'loose-silent'
+  if (process.env.TURBOPACK === 'auto') {
+    // If next has defaulted to turbopack, override it.
+    delete process.env.TURBOPACK
+  } else {
+    console.error(
+      `Cannot call withRspack and pass the ${process.env.TURBOPACK ? '--turbopack' : '--webpack'} flag.`
+    )
+    console.error('Please configure only one bundler.')
+    process.exit(1)
+  }
   return config
 }
diff --git a/packages/next/src/bin/next.ts b/packages/next/src/bin/next.ts
index cfcb4b5b355b72..a947d1ad95aeed 100755
--- a/packages/next/src/bin/next.ts
+++ b/packages/next/src/bin/next.ts
@@ -133,8 +133,9 @@ program
   .option('--no-mangling', 'Disables mangling.')
   .option('--profile', 'Enables production profiling for React.')
   .option('--experimental-app-only', 'Builds only App Router routes.')
-  .option('--turbo', 'Starts development mode using Turbopack.')
-  .option('--turbopack', 'Starts development mode using Turbopack.')
+  .option('--turbo', 'Builds using Turbopack.')
+  .option('--turbopack', 'Builds using Turbopack.')
+  .option('--webpack', 'Builds using webpack.')
   .addOption(
     new Option(
       '--experimental-build-mode [mode]',
@@ -173,6 +174,7 @@ program
   )
   .option('--turbo', 'Starts development mode using Turbopack.')
   .option('--turbopack', 'Starts development mode using Turbopack.')
+  .option('--webpack', 'Starts development mode using webpack.')
   .addOption(
     new Option(
       '-p, --port <port>',
diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 9225b231b9d6ff..5394a9b2bd5938 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -83,6 +83,7 @@ import {
   UNDERSCORE_GLOBAL_ERROR_ROUTE_ENTRY,
 } from '../shared/lib/entry-constants'
 import { isDynamicRoute } from '../shared/lib/router/utils'
+import { Bundler, finalizeBundlerFromConfig } from '../lib/bundler'
 import type { __ApiPreviewProps } from '../server/api-utils'
 import loadConfig from '../server/config'
 import type { BuildManifest } from '../server/get-page-files'
@@ -901,7 +902,7 @@ export default async function build(
   runLint = true,
   noMangling = false,
   appDirOnly = false,
-  isTurbopack = false,
+  bundler = Bundler.Turbopack,
   experimentalBuildMode: 'default' | 'compile' | 'generate' | 'generate-env',
   traceUploadUrl: string | undefined
 ): Promise<void> {
@@ -914,7 +915,6 @@ export default async function build(
   try {
     const nextBuildSpan = trace('next-build', undefined, {
       buildMode: experimentalBuildMode,
-      isTurboBuild: String(isTurbopack),
       version: process.env.__NEXT_VERSION as string,
     })
 
@@ -949,6 +949,10 @@ export default async function build(
         )
       loadedConfig = config
 
+      // Reading the config can modify environment variables that influence the bundler selection.
+      bundler = finalizeBundlerFromConfig(bundler)
+      nextBuildSpan.setAttribute('bundler', getBundlerForTelemetry(bundler))
+
       process.env.NEXT_DEPLOYMENT_ID = config.deploymentId || ''
       NextBuildContext.config = config
 
@@ -971,7 +975,7 @@ export default async function build(
       NextBuildContext.buildId = buildId
 
       if (experimentalBuildMode === 'generate-env') {
-        if (isTurbopack) {
+        if (bundler === Bundler.Turbopack) {
           Log.warn('generate-env is not needed with turbopack')
           process.exit(0)
         }
@@ -1398,7 +1402,7 @@ export default async function build(
         })
 
       // Turbopack already handles conflicting app and page routes.
-      if (!isTurbopack) {
+      if (bundler !== Bundler.Turbopack) {
         const numConflictingAppPaths = conflictingAppPagePaths.length
         if (mappedAppPages && numConflictingAppPaths > 0) {
           Log.error(
@@ -1681,7 +1685,7 @@ export default async function build(
 
       let shutdownPromise = Promise.resolve()
       if (!isGenerateMode) {
-        if (isTurbopack) {
+        if (bundler === Bundler.Turbopack) {
           const {
             duration: compilerDuration,
             shutdownPromise: p,
@@ -1795,7 +1799,7 @@ export default async function build(
 
             telemetry.record(
               eventBuildCompleted(pagesPaths, {
-                bundler: getBundlerForTelemetry(isTurbopack),
+                bundler: getBundlerForTelemetry(bundler),
                 durationInSeconds,
                 totalAppPagesCount,
               })
@@ -1811,7 +1815,7 @@ export default async function build(
 
             telemetry.record(
               eventBuildCompleted(pagesPaths, {
-                bundler: getBundlerForTelemetry(isTurbopack),
+                bundler: getBundlerForTelemetry(bundler),
                 durationInSeconds: compilerDuration,
                 totalAppPagesCount,
               })
@@ -2429,7 +2433,10 @@ export default async function build(
         )
         // If there's edge routes, append the edge instrumentation hook
         // Turbopack generates this chunk with a hashed name and references it in middleware-manifest.
-        if (!isTurbopack && (edgeRuntimeAppCount || edgeRuntimePagesCount)) {
+        if (
+          bundler !== Bundler.Turbopack &&
+          (edgeRuntimeAppCount || edgeRuntimePagesCount)
+        ) {
           instrumentationHookEntryFiles.push(
             path.join(
               SERVER_DIRECTORY,
@@ -2482,7 +2489,7 @@ export default async function build(
               path.join(SERVER_DIRECTORY, FUNCTIONS_CONFIG_MANIFEST),
               path.join(SERVER_DIRECTORY, MIDDLEWARE_MANIFEST),
               path.join(SERVER_DIRECTORY, MIDDLEWARE_BUILD_MANIFEST + '.js'),
-              ...(!isTurbopack
+              ...(bundler !== Bundler.Turbopack
                 ? [
                     path.join(
                       SERVER_DIRECTORY,
@@ -2517,7 +2524,7 @@ export default async function build(
                     ),
                   ]
                 : []),
-              ...(pagesDir && !isTurbopack
+              ...(pagesDir && bundler !== Bundler.Turbopack
                 ? [
                     DYNAMIC_CSS_MANIFEST + '.json',
                     path.join(SERVER_DIRECTORY, DYNAMIC_CSS_MANIFEST + '.js'),
@@ -2569,7 +2576,7 @@ export default async function build(
             ],
           }
 
-          if (isTurbopack) {
+          if (bundler === Bundler.Turbopack) {
             await writeManifest(
               path.join(
                 distDir,
@@ -2585,7 +2592,11 @@ export default async function build(
 
       await writeFunctionsConfigManifest(distDir, functionsConfigManifest)
 
-      if (!isTurbopack && !isGenerateMode && !buildTracesPromise) {
+      if (
+        bundler !== Bundler.Turbopack &&
+        !isGenerateMode &&
+        !buildTracesPromise
+      ) {
         buildTracesPromise = nextBuildSpan
           .traceChild('collect-build-traces')
           .traceAsyncFn(() => {
@@ -2726,7 +2737,7 @@ export default async function build(
 
       // we don't need to inline for turbopack build as
       // it will handle it's own caching separate of compile
-      if (isGenerateMode && !isTurbopack) {
+      if (isGenerateMode && bundler !== Bundler.Turbopack) {
         Log.info('Inlining static env ...')
 
         await nextBuildSpan
@@ -4132,7 +4143,7 @@ export default async function build(
     if (telemetry) {
       telemetry.record(
         eventBuildFailed({
-          bundler: getBundlerForTelemetry(isTurbopack),
+          bundler: getBundlerForTelemetry(bundler),
           errorCode: getErrorCodeForTelemetry(e),
           durationInSeconds: Math.floor((Date.now() - buildStartTime) / 1000),
         })
@@ -4153,7 +4164,7 @@ export default async function build(
         mode: 'build',
         projectDir: dir,
         distDir: loadedConfig.distDir,
-        isTurboSession: isTurbopack,
+        isTurboSession: bundler === Bundler.Turbopack,
         sync: true,
       })
     }
@@ -4167,16 +4178,17 @@ function errorFromUnsupportedSegmentConfig(): never {
   process.exit(1)
 }
 
-function getBundlerForTelemetry(isTurbopack: boolean) {
-  if (isTurbopack) {
-    return 'turbopack'
+function getBundlerForTelemetry(bundler: Bundler) {
+  switch (bundler) {
+    case Bundler.Turbopack:
+      return 'turbopack'
+    case Bundler.Rspack:
+      return 'rspack'
+    case Bundler.Webpack:
+      return 'webpack'
+    default:
+      throw new Error(`unknown bundler: ${bundler}`)
   }
-
-  if (process.env.NEXT_RSPACK) {
-    return 'rspack'
-  }
-
-  return 'webpack'
 }
 
 function getErrorCodeForTelemetry(err: unknown) {
diff --git a/packages/next/src/cli/next-build.ts b/packages/next/src/cli/next-build.ts
index a2df58fdc41356..bea9c4900e8e68 100755
--- a/packages/next/src/cli/next-build.ts
+++ b/packages/next/src/cli/next-build.ts
@@ -10,6 +10,7 @@ import isError from '../lib/is-error'
 import { getProjectDir } from '../lib/get-project-dir'
 import { enableMemoryDebuggingMode } from '../lib/memory/startup'
 import { disableMemoryDebuggingMode } from '../lib/memory/shutdown'
+import { parseBundlerArgs } from '../lib/bundler'
 
 export type NextBuildOptions = {
   debug?: boolean
@@ -19,6 +20,7 @@ export type NextBuildOptions = {
   mangling: boolean
   turbo?: boolean
   turbopack?: boolean
+  webpack?: boolean
   experimentalDebugMemoryUsage: boolean
   experimentalAppOnly?: boolean
   experimentalTurbo?: boolean
@@ -82,12 +84,7 @@ const nextBuild = (options: NextBuildOptions, directory?: string) => {
     printAndExit(`> No such directory exists as the project root: ${dir}`)
   }
 
-  const isTurbopack = Boolean(
-    options.turbo || options.turbopack || process.env.IS_TURBOPACK_TEST
-  )
-  if (isTurbopack) {
-    process.env.TURBOPACK = '1'
-  }
+  const bundler = parseBundlerArgs(options)
 
   return build(
     dir,
@@ -97,7 +94,7 @@ const nextBuild = (options: NextBuildOptions, directory?: string) => {
     lint,
     !mangling,
     experimentalAppOnly,
-    isTurbopack,
+    bundler,
     experimentalBuildMode,
     traceUploadUrl
   )
diff --git a/packages/next/src/cli/next-dev.ts b/packages/next/src/cli/next-dev.ts
index 33e6e51bc6d881..b4f1e0db71f6dc 100644
--- a/packages/next/src/cli/next-dev.ts
+++ b/packages/next/src/cli/next-dev.ts
@@ -38,11 +38,17 @@ import { once } from 'node:events'
 import { clearTimeout } from 'timers'
 import { flushAllTraces, trace } from '../trace'
 import { traceId } from '../trace/shared'
+import {
+  Bundler,
+  finalizeBundlerFromConfig,
+  parseBundlerArgs,
+} from '../lib/bundler'
 
 export type NextDevOptions = {
   disableSourceMaps: boolean
   turbo?: boolean
   turbopack?: boolean
+  webpack?: boolean
   port: number
   hostname?: string
   experimentalHttps?: boolean
@@ -58,11 +64,11 @@ let dir: string
 let child: undefined | ChildProcess
 // The config in next-dev is only used to access config.distDir for telemetry and trace.
 let config: NextConfigComplete
-let isTurboSession = false
+let bundler: Bundler
 let traceUploadUrl: string
 let sessionStopHandled = false
-let sessionStarted = Date.now()
-let sessionSpan = trace('next-dev')
+const sessionStarted = Date.now()
+const sessionSpan = trace('next-dev')
 
 // How long should we wait for the child to cleanly exit after sending
 // SIGINT/SIGTERM to the child process before sending SIGKILL?
@@ -123,11 +129,13 @@ const handleSessionStop = async (signal: NodeJS.Signals | number | null) => {
       new Telemetry({
         distDir: path.join(dir, config.distDir),
       })
+    // Reading the config can modify environment variables that influence the bundler selection.
+    bundler = finalizeBundlerFromConfig(bundler)
 
     telemetry.record(
       eventCliSessionStopped({
         cliCommand: 'dev',
-        turboFlag: isTurboSession,
+        turboFlag: bundler === Bundler.Turbopack,
         durationMilliseconds: Date.now() - sessionStarted,
         pagesDir,
         appDir,
@@ -146,7 +154,7 @@ const handleSessionStop = async (signal: NodeJS.Signals | number | null) => {
       mode: 'dev',
       projectDir: dir,
       distDir: config.distDir,
-      isTurboSession,
+      isTurboSession: bundler === Bundler.Turbopack,
     })
   }
 
@@ -168,14 +176,7 @@ const nextDev = async (
   portSource: PortSource,
   directory?: string
 ) => {
-  const isTurbopack = Boolean(
-    options.turbo || options.turbopack || process.env.IS_TURBOPACK_TEST
-  )
-  if (isTurbopack) {
-    process.env.TURBOPACK = '1'
-  }
-
-  isTurboSession = isTurbopack
+  bundler = parseBundlerArgs(options)
 
   dir = getProjectDir(process.env.NEXT_PRIVATE_DEV_DIR || directory)
 
@@ -288,7 +289,9 @@ const nextDev = async (
         stdio: 'inherit',
         env: {
           ...defaultEnv,
-          ...(isTurbopack ? { TURBOPACK: '1' } : undefined),
+          ...(bundler === Bundler.Turbopack
+            ? { TURBOPACK: process.env.TURBOPACK }
+            : undefined),
           NEXT_PRIVATE_WORKER: '1',
           NEXT_PRIVATE_TRACE_ID: traceId,
           NODE_EXTRA_CA_CERTS: startServerOptions.selfSignedCertificate
@@ -336,12 +339,13 @@ const nextDev = async (
               (await loadConfig(PHASE_DEVELOPMENT_SERVER, dir, {
                 silent: true,
               }))
+            bundler = finalizeBundlerFromConfig(bundler)
             uploadTrace({
               traceUploadUrl,
               mode: 'dev',
               projectDir: dir,
               distDir: config.distDir,
-              isTurboSession,
+              isTurboSession: bundler === Bundler.Turbopack,
               sync: true,
             })
           }
diff --git a/packages/next/src/lib/bundler.ts b/packages/next/src/lib/bundler.ts
new file mode 100644
index 00000000000000..38eee0866d2e6f
--- /dev/null
+++ b/packages/next/src/lib/bundler.ts
@@ -0,0 +1,99 @@
+/// Utilties for configuring the bundler to use.
+export enum Bundler {
+  Turbopack,
+  Webpack,
+  Rspack,
+}
+/**
+ * Parse the bundler arguments and potentially sets the `TURBOPACK` environment variable.
+ *
+ * NOTE: rspack is configured via next config which is chaotic so it is possible for this to be overridden later.
+ *
+ * @param options The options to parse.
+ * @returns The bundler that was configured
+ */
+export function parseBundlerArgs(options: {
+  turbo?: boolean
+  turbopack?: boolean
+  webpack?: boolean
+}): Bundler {
+  const bundlerFlags = new Map<Bundler, string[]>()
+  const setBundlerFlag = (bundler: Bundler, flag: string) => {
+    bundlerFlags.set(bundler, (bundlerFlags.get(bundler) ?? []).concat(flag))
+  }
+  // What turbo flag was set? We allow multiple to be set, which is silly but not ambiguous, just pick the most relevant one.
+  if (options.turbopack) {
+    setBundlerFlag(Bundler.Turbopack, '--turbopack')
+  }
+  if (options.turbo) {
+    setBundlerFlag(Bundler.Turbopack, '--turbo')
+  } else if (process.env.TURBOPACK) {
+    // We don't really want to support this but it is trivial and not really confusing.
+    // If we don't support it and someone sets it, we would have inconsistent behavior
+    // since some parts of next would read the return value of this function and other
+    // parts will read the env variable.
+    setBundlerFlag(Bundler.Turbopack, `TURBOPACK=${process.env.TURBOPACK}`)
+  } else if (process.env.IS_TURBOPACK_TEST) {
+    setBundlerFlag(
+      Bundler.Turbopack,
+      `IS_TURBOPACK_TEST=${process.env.IS_TURBOPACK_TEST}`
+    )
+  }
+  if (options.webpack) {
+    setBundlerFlag(Bundler.Webpack, '--webpack')
+  }
+
+  if (process.env.IS_WEBPACK_TEST) {
+    setBundlerFlag(
+      Bundler.Webpack,
+      `IS_WEBPACK_TEST=${process.env.IS_WEBPACK_TEST}`
+    )
+  }
+
+  // Mostly this is set via the NextConfig but it can also be set via the command line which is
+  // common for testing.
+  if (process.env.NEXT_RSPACK) {
+    setBundlerFlag(Bundler.Rspack, `NEXT_RSPACK=${process.env.NEXT_RSPACK}`)
+  }
+  if (process.env.NEXT_TEST_USE_RSPACK) {
+    setBundlerFlag(
+      Bundler.Rspack,
+      `NEXT_TEST_USE_RSPACK=${process.env.NEXT_TEST_USE_RSPACK}`
+    )
+  }
+
+  if (bundlerFlags.size > 1) {
+    console.error(
+      `Multiple bundler flags set: ${Array.from(bundlerFlags.values()).flat().join(', ')}. Configure exactly one bundler.`
+    )
+    process.exit(1)
+  }
+  // The default is turbopack when nothing is configured.
+  if (bundlerFlags.size === 0) {
+    process.env.TURBOPACK = 'auto'
+    return Bundler.Turbopack
+  }
+  if (bundlerFlags.has(Bundler.Turbopack)) {
+    // Only conditionally assign to the environment variable, preserving already set values.
+    // If it was set to 'auto' because no flag was set and this function is called a second time we
+    // would upgrade to '1' but we don't really want that.
+    process.env.TURBOPACK ??= '1'
+    return Bundler.Turbopack
+  }
+  // Otherwise it is one of rspack or webpack. At this point there must be exactly one key in the map.
+  return bundlerFlags.keys().next().value!
+}
+
+/**
+ * Finalize the bundler based on the config.
+ *
+ * Rspack is configured via next config by setting an environment variable (yay, side effects)
+ * so this should only be called after parsing the config.
+ */
+export function finalizeBundlerFromConfig(fromOptions: Bundler) {
+  // Reading the next config can set NEXT_RSPACK environment variables.
+  if (process.env.NEXT_RSPACK) {
+    return Bundler.Rspack
+  }
+  return fromOptions
+}
diff --git a/packages/next/src/lib/turbopack-warning.ts b/packages/next/src/lib/turbopack-warning.ts
index d737c3505a49a6..4858c0265b3dd4 100644
--- a/packages/next/src/lib/turbopack-warning.ts
+++ b/packages/next/src/lib/turbopack-warning.ts
@@ -1,4 +1,4 @@
-import type { NextConfig } from '../server/config-shared'
+import type { NextConfigComplete } from '../server/config-shared'
 import loadConfig from '../server/config'
 import * as Log from '../build/output/log'
 import {
@@ -43,11 +43,9 @@ const unsupportedTurbopackNextConfigOptions = [
 ]
 
 // The following will need to be supported by `next build --turbopack`
-const unsupportedProductionSpecificTurbopackNextConfigOptions: string[] = [
-  // TODO: Support disabling sourcemaps, currently they're always enabled.
-  // 'productionBrowserSourceMaps',
-]
+const unsupportedProductionSpecificTurbopackNextConfigOptions: string[] = []
 
+/**  */
 export async function validateTurboNextConfig({
   dir,
   isDev,
@@ -67,8 +65,8 @@ export async function validateTurboNextConfig({
   let hasWebpackConfig = false
   let hasTurboConfig = false
 
-  let unsupportedConfig: string[] = []
-  let rawNextConfig: NextConfig = {}
+  const unsupportedConfig: string[] = []
+  let rawNextConfig: NextConfigComplete = {} as NextConfigComplete
 
   const phase = isDev ? PHASE_DEVELOPMENT_SERVER : PHASE_PRODUCTION_BUILD
   try {
@@ -76,7 +74,7 @@ export async function validateTurboNextConfig({
       await loadConfig(phase, dir, {
         rawConfig: true,
       })
-    ) as NextConfig
+    )
 
     if (typeof rawNextConfig === 'function') {
       rawNextConfig = (rawNextConfig as any)(phase, {
@@ -120,7 +118,7 @@ export async function validateTurboNextConfig({
 
     const customKeys = flattenKeys(rawNextConfig)
 
-    let unsupportedKeys = isDev
+    const unsupportedKeys = isDev
       ? unsupportedTurbopackNextConfigOptions
       : [
           ...unsupportedTurbopackNextConfigOptions,
@@ -135,7 +133,7 @@ export async function validateTurboNextConfig({
         hasTurboConfig = true
       }
 
-      let isUnsupported =
+      const isUnsupported =
         unsupportedKeys.some(
           (unsupportedKey) =>
             // Either the key matches (or is a more specific subkey) of
@@ -158,13 +156,24 @@ export async function validateTurboNextConfig({
     Log.error('Unexpected error occurred while checking config', e)
   }
 
-  if (hasWebpackConfig && !hasTurboConfig) {
-    Log.warn(
-      `Webpack is configured while Turbopack is not, which may cause problems.`
-    )
-    Log.warn(
-      `See instructions if you need to configure Turbopack:\n  https://nextjs.org/docs/app/api-reference/next-config-js/turbopack\n`
+  // If the build was defaulted to Turbopack, we want to warn about possibly ignored webpack
+  // configuration. Otherwise the user explicitly picked turbopack and thus we expect that
+  // they have configured it correctly.
+  if (process.env.TURBOPACK === 'auto' && hasWebpackConfig && !hasTurboConfig) {
+    const configFile = rawNextConfig.configFileName ?? 'your Next config file'
+    Log.error(
+      `ERROR: This build is using Turbopack, with a \`webpack\` config and no \`turbopack\` config. This may be a mistake.
+
+  As of Next.js 16 turbopack is enabled by default and custom webpack configurations may need to be migrated to Turbopack.
+
+  NOTE: your \`webpack\` config may have been added by a configuration plugin.
+
+  To configure Turbopack, see https://nextjs.org/docs/app/api-reference/next-config-js/turbopack
+
+  TIP: Many applications work fine under Turbopack with no configuration, if that is the case for you, you can silence this error by passing the \`--turbopack\` or \`--webpack\` flag explicitly or simply setting an empty turbopack config in ${configFile} (e.g. \`turbopack: {}\`).`
     )
+
+    process.exit(1)
   }
 
   if (unsupportedConfig.length) {
diff --git a/packages/next/src/server/next.ts b/packages/next/src/server/next.ts
index 88de1cd32a45d3..2ca7d7b9bffb9a 100644
--- a/packages/next/src/server/next.ts
+++ b/packages/next/src/server/next.ts
@@ -540,7 +540,8 @@ function createServer(
     options &&
     (options.turbo || options.turbopack || process.env.IS_TURBOPACK_TEST)
   ) {
-    process.env.TURBOPACK = '1'
+    // Configure TURBOPACK if it isn't already set
+    process.env.TURBOPACK ??= '1'
   }
   // The package is used as a TypeScript plugin.
   if (
diff --git a/scripts/test-new-tests.mjs b/scripts/test-new-tests.mjs
index f62b101ea0f9e5..881ae9dc3b21a7 100644
--- a/scripts/test-new-tests.mjs
+++ b/scripts/test-new-tests.mjs
@@ -153,6 +153,7 @@ async function main() {
         NEXT_EXTERNAL_TESTS_FILTERS,
         NEXT_TEST_MODE: testMode,
         NEXT_TEST_VERSION: nextTestVersion,
+        IS_WEBPACK_TEST: '1',
       },
     })
   }
@@ -170,6 +171,7 @@ async function main() {
           NEXT_TEST_VERSION: nextTestVersion,
           IS_TURBOPACK_TEST: '1',
           TURBOPACK_BUILD: testMode === 'start' ? '1' : undefined,
+          TURBOPACK_DEV: testMode === 'dev' ? '1' : undefined,
         },
       })
     }
diff --git a/test/development/next-config-ts/turbo/index.test.ts b/test/development/next-config-ts/turbo/index.test.ts
index f43c75d6176fdf..348568ae507ed3 100644
--- a/test/development/next-config-ts/turbo/index.test.ts
+++ b/test/development/next-config-ts/turbo/index.test.ts
@@ -1,13 +1,15 @@
 import { nextTestSetup } from 'e2e-utils'
-
-describe('next-config-ts - turbopack', () => {
-  const { next } = nextTestSetup({
-    files: __dirname,
-    // explicitly ensure that turbopack is used
-    startCommand: 'pnpm next dev --turbo',
-  })
-  it('should work with Turbopack', async () => {
-    const $ = await next.render$('/')
-    expect($('p').text()).toBe('foo')
-  })
-})
+;(process.env.IS_TURBOPACK_TEST ? describe : describe.skip)(
+  'next-config-ts - turbopack',
+  () => {
+    const { next } = nextTestSetup({
+      files: __dirname,
+      // explicitly ensure that turbopack is used
+      startCommand: 'pnpm next dev --turbopack',
+    })
+    it('should work with Turbopack', async () => {
+      const $ = await next.render$('/')
+      expect($('p').text()).toBe('foo')
+    })
+  }
+)
diff --git a/test/e2e/config-turbopack/index.test.ts b/test/e2e/config-turbopack/index.test.ts
index 36dacfd63a972f..64089fd3d01298 100644
--- a/test/e2e/config-turbopack/index.test.ts
+++ b/test/e2e/config-turbopack/index.test.ts
@@ -1,96 +1,142 @@
 /* eslint-disable jest/no-standalone-expect */
 import { nextTestSetup } from 'e2e-utils'
 
-const WARNING_MESSAGE = 'Webpack is configured while Turbopack is not'
+const WARNING_MESSAGE =
+  'ERROR: This build is using Turbopack, with a `webpack` config and no `turbopack` config. This may be a mistake'
 
 const itif = (condition: boolean) => (condition ? it : it.skip)
 
-describe('config-turbopack', () => {
-  describe('when webpack is configured but Turbopack is not', () => {
-    const { next, isTurbopack } = nextTestSetup({
-      files: {
-        'app/page.js': `
-          export default function Page() {
-            return <p>hello world</p>
-          }
-        `,
-        'next.config.js': `
+const page = {
+  'app/page.js': `
+export default function Page() {
+  return <p>hello world</p>
+}
+`,
+}
+
+;(process.env.IS_TURBOPACK_TEST ? describe : describe.skip)(
+  'config-turbopack',
+  () => {
+    describe('when turbopack is auto selected', () => {
+      describe('when webpack is configured but Turbopack is not', () => {
+        const { next, isNextDev, isNextStart } = nextTestSetup({
+          skipStart: Boolean(process.env.NEXT_TEST_MODE === 'start'),
+          turbo: false,
+          env: {
+            TURBOPACK: 'auto',
+          },
+          files: {
+            ...page,
+            'next.config.js': `
           module.exports = {
             webpack: (config) => {
               return config
             },
           }
         `,
-      },
-    })
+          },
+        })
+
+        itif(isNextDev)('warns', async () => {
+          if (next)
+            try {
+              await next.render('/')
+            } catch (e) {
+              // we expect an error but this is the only way to get the server to crash
+            }
 
-    itif(isTurbopack)('warns', async () => {
-      if (next) await next.render('/')
-      expect(next.cliOutput).toContain(WARNING_MESSAGE)
+          expect(next.cliOutput).toContain(WARNING_MESSAGE)
+        })
+        itif(isNextStart)('errors', async () => {
+          const { exitCode, cliOutput } = await next.build()
+          expect(exitCode).toBe(1)
+          expect(cliOutput).toContain(WARNING_MESSAGE)
+        })
+      })
+      // no warn cases work when auto selected too
+      noWarnCases()
     })
-  })
 
-  describe('when webpack is configured and config.turbopack is set', () => {
-    const { next, isTurbopack } = nextTestSetup({
-      files: {
-        'app/page.js': `
-          export default function Page() {
-            return <p>hello world</p>
-          }
-        `,
-        'next.config.js': `
-          module.exports = {
-            turbopack: {
-              rules: {
-                '*.foo': {
-                  loaders: ['foo-loader']
-                }
+    describe('when turbopack is explicitly configured', () => {
+      describe('when webpack is configured but Turbopack is not', () => {
+        const { next } = nextTestSetup({
+          files: {
+            ...page,
+            'next.config.js': `
+              module.exports = {
+                webpack: (config) => {
+                  return config
+                },
               }
-            },
-            webpack: (config) => {
-              return config
-            },
-          }
-        `,
-      },
-    })
+            `,
+          },
+        })
 
-    itif(isTurbopack)('does not warn', async () => {
-      if (next) await next.render('/')
-      expect(next.cliOutput).not.toContain(WARNING_MESSAGE)
+        it('does not warn', async () => {
+          if (next) await next.render('/')
+          expect(next.cliOutput).not.toContain(WARNING_MESSAGE)
+        })
+      })
+      noWarnCases()
     })
-  })
-
-  describe('when webpack is configured and config.experimental.turbo is set', () => {
-    const { next, isTurbopack } = nextTestSetup({
-      files: {
-        'app/page.js': `
-          export default function Page() {
-            return <p>hello world</p>
-          }
-        `,
-        'next.config.js': `
-          module.exports = {
-            experimental: {
-              turbo: {
+    /// These other cases don't warn because --turbopack is explicitly selected
+    function noWarnCases(env?: Record<string, string>) {
+      describe('when webpack is configured and config.turbopack is set', () => {
+        const { next } = nextTestSetup({
+          env,
+          files: {
+            ...page,
+            'next.config.js': `
+            module.exports = {
+              turbopack: {
                 rules: {
                   '*.foo': {
                     loaders: ['foo-loader']
                   }
                 }
-              }
-            },
-            webpack: (config) => {
-              return config
-            },
-          }
-        `,
-      },
-    })
+              },
+              webpack: (config) => {
+                return config
+              },
+            }
+          `,
+          },
+        })
 
-    itif(isTurbopack)('does not warn', async () => {
-      if (next) await next.render('/')
-      expect(next.cliOutput).not.toContain(WARNING_MESSAGE)
-    })
-  })
-})
+        it('does not warn', async () => {
+          if (next) await next.render('/')
+          expect(next.cliOutput).not.toContain(WARNING_MESSAGE)
+        })
+      })
+
+      describe('when webpack is configured and config.experimental.turbo is set', () => {
+        const { next } = nextTestSetup({
+          files: {
+            ...page,
+            'next.config.js': `
+            module.exports = {
+              experimental: {
+                turbo: {
+                  rules: {
+                    '*.foo': {
+                      loaders: ['foo-loader']
+                    }
+                  }
+                }
+              },
+              webpack: (config) => {
+                return config
+              },
+            }
+          `,
+          },
+        })
+
+        it('does not warn', async () => {
+          if (next) await next.render('/')
+          expect(next.cliOutput).not.toContain(WARNING_MESSAGE)
+        })
+      })
+    }
+  }
+)