diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index b59921c29489b..7dec5d3382c61 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2685,6 +2685,10 @@
         "category": "Message",
         "code": 6080
     },
+    "File '{0}' has an unsupported extension, so skipping it.": {
+        "category": "Message",
+        "code": 6081
+    },
     "Only 'amd' and 'system' modules are supported alongside --{0}.": {
         "category": "Error",
         "code": 6082
diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index 0daca9156d1fc..4291f05f24bc3 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -47,11 +47,6 @@ namespace ts {
         return resolved.path;
     }
 
-    /** Create Resolved from a file with unknown extension. */
-    function resolvedFromAnyFile(path: string): Resolved | undefined {
-        return { path, extension: extensionFromPath(path) };
-    }
-
     /** Adds `isExernalLibraryImport` to a Resolved to get a ResolvedModule. */
     function resolvedModuleFromResolved({ path, extension }: Resolved, isExternalLibraryImport: boolean): ResolvedModuleFull {
         return { resolvedFileName: path, extension, isExternalLibraryImport };
@@ -71,7 +66,8 @@ namespace ts {
         traceEnabled: boolean;
     }
 
-    function tryReadTypesSection(extensions: Extensions, packageJsonPath: string, baseDirectory: string, state: ModuleResolutionState): string {
+    /** Reads from "main" or "types"/"typings" depending on `extensions`. */
+    function tryReadPackageJsonMainOrTypes(extensions: Extensions, packageJsonPath: string, baseDirectory: string, state: ModuleResolutionState): string {
         const jsonContent = readJson(packageJsonPath, state.host);
 
         switch (extensions) {
@@ -678,18 +674,21 @@ namespace ts {
             if (state.traceEnabled) {
                 trace(state.host, Diagnostics.Found_package_json_at_0, packageJsonPath);
             }
-            const typesFile = tryReadTypesSection(extensions, packageJsonPath, candidate, state);
-            if (typesFile) {
-                const onlyRecordFailures = !directoryProbablyExists(getDirectoryPath(typesFile), state.host);
+            const mainOrTypesFile = tryReadPackageJsonMainOrTypes(extensions, packageJsonPath, candidate, state);
+            if (mainOrTypesFile) {
+                const onlyRecordFailures = !directoryProbablyExists(getDirectoryPath(mainOrTypesFile), state.host);
                 // A package.json "typings" may specify an exact filename, or may choose to omit an extension.
-                const fromFile = tryFile(typesFile, failedLookupLocations, onlyRecordFailures, state);
-                if (fromFile) {
-                    // Note: this would allow a package.json to specify a ".js" file as typings. Maybe that should be forbidden.
-                    return resolvedFromAnyFile(fromFile);
+                const fromExactFile = tryFile(mainOrTypesFile, failedLookupLocations, onlyRecordFailures, state);
+                if (fromExactFile) {
+                    const resolved = fromExactFile && resolvedIfExtensionMatches(extensions, fromExactFile);
+                    if (resolved) {
+                        return resolved;
+                    }
+                    trace(state.host, Diagnostics.File_0_has_an_unsupported_extension_so_skipping_it, fromExactFile);
                 }
-                const x = tryAddingExtensions(typesFile, Extensions.TypeScript, failedLookupLocations, onlyRecordFailures, state);
-                if (x) {
-                    return x;
+                const resolved = tryAddingExtensions(mainOrTypesFile, Extensions.TypeScript, failedLookupLocations, onlyRecordFailures, state);
+                if (resolved) {
+                    return resolved;
                 }
             }
             else {
@@ -709,6 +708,24 @@ namespace ts {
         return loadModuleFromFile(extensions, combinePaths(candidate, "index"), failedLookupLocations, !directoryExists, state);
     }
 
+    /** Resolve from an arbitrarily specified file. Return `undefined` if it has an unsupported extension. */
+    function resolvedIfExtensionMatches(extensions: Extensions, path: string): Resolved | undefined {
+        const extension = tryGetExtensionFromPath(path);
+        return extension !== undefined && extensionIsOk(extensions, extension) ? { path, extension } : undefined;
+    }
+
+    /** True if `extension` is one of the supported `extensions`. */
+    function extensionIsOk(extensions: Extensions, extension: Extension): boolean {
+        switch (extensions) {
+            case Extensions.JavaScript:
+                return extension === Extension.Js || extension === Extension.Jsx;
+            case Extensions.TypeScript:
+                return extension === Extension.Ts || extension === Extension.Tsx || extension === Extension.Dts;
+            case Extensions.DtsOnly:
+                return extension === Extension.Dts;
+        }
+    }
+
     function pathToPackageJson(directory: string): string {
         return combinePaths(directory, "package.json");
     }
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.js b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.js
new file mode 100644
index 0000000000000..b5d4ae20a4db7
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.js
@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/moduleResolutionWithExtensions_unexpected.ts] ////
+
+//// [normalize.css]
+// This tests that a package.json "main" with an unexpected extension is ignored.
+
+This file is not read.
+
+//// [package.json]
+{ "main": "normalize.css" }
+
+//// [a.ts]
+import "normalize.css";
+
+
+//// [a.js]
+"use strict";
+require("normalize.css");
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.symbols b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.symbols
new file mode 100644
index 0000000000000..a73d58fd7d8e9
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.symbols
@@ -0,0 +1,4 @@
+=== /a.ts ===
+import "normalize.css";
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.trace.json b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.trace.json
new file mode 100644
index 0000000000000..84532e8db7b22
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.trace.json
@@ -0,0 +1,29 @@
+[
+    "======== Resolving module 'normalize.css' from '/a.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module 'normalize.css' from 'node_modules' folder.",
+    "File '/node_modules/normalize.css.ts' does not exist.",
+    "File '/node_modules/normalize.css.tsx' does not exist.",
+    "File '/node_modules/normalize.css.d.ts' does not exist.",
+    "Found 'package.json' at '/node_modules/normalize.css/package.json'.",
+    "'package.json' does not have a 'types' or 'main' field.",
+    "File '/node_modules/normalize.css/index.ts' does not exist.",
+    "File '/node_modules/normalize.css/index.tsx' does not exist.",
+    "File '/node_modules/normalize.css/index.d.ts' does not exist.",
+    "File '/node_modules/@types/normalize.css.d.ts' does not exist.",
+    "File '/node_modules/@types/normalize.css/package.json' does not exist.",
+    "File '/node_modules/@types/normalize.css/index.d.ts' does not exist.",
+    "Loading module 'normalize.css' from 'node_modules' folder.",
+    "File '/node_modules/normalize.css.js' does not exist.",
+    "File '/node_modules/normalize.css.jsx' does not exist.",
+    "Found 'package.json' at '/node_modules/normalize.css/package.json'.",
+    "No types specified in 'package.json', so returning 'main' value of 'normalize.css'",
+    "File '/node_modules/normalize.css/normalize.css' exist - use it as a name resolution result.",
+    "File '/node_modules/normalize.css/normalize.css' has an unsupported extension, so skipping it.",
+    "File '/node_modules/normalize.css/normalize.css.ts' does not exist.",
+    "File '/node_modules/normalize.css/normalize.css.tsx' does not exist.",
+    "File '/node_modules/normalize.css/normalize.css.d.ts' does not exist.",
+    "File '/node_modules/normalize.css/index.js' does not exist.",
+    "File '/node_modules/normalize.css/index.jsx' does not exist.",
+    "======== Module name 'normalize.css' was not resolved. ========"
+]
\ No newline at end of file
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.types b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.types
new file mode 100644
index 0000000000000..a73d58fd7d8e9
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected.types
@@ -0,0 +1,4 @@
+=== /a.ts ===
+import "normalize.css";
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.js b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.js
new file mode 100644
index 0000000000000..dc9df79e4dfb7
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.js
@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/moduleResolutionWithExtensions_unexpected2.ts] ////
+
+//// [foo.js]
+// This tests that a package.json "types" with an unexpected extension is ignored.
+
+This file is not read.
+
+//// [package.json]
+{ "types": "foo.js" }
+
+//// [a.ts]
+import "foo";
+
+
+//// [a.js]
+"use strict";
+require("foo");
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.symbols b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.symbols
new file mode 100644
index 0000000000000..11c9c72cc6907
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.symbols
@@ -0,0 +1,4 @@
+=== /a.ts ===
+import "foo";
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.trace.json b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.trace.json
new file mode 100644
index 0000000000000..27c9d2243e5f2
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.trace.json
@@ -0,0 +1,29 @@
+[
+    "======== Resolving module 'foo' from '/a.ts'. ========",
+    "Module resolution kind is not specified, using 'NodeJs'.",
+    "Loading module 'foo' from 'node_modules' folder.",
+    "File '/node_modules/foo.ts' does not exist.",
+    "File '/node_modules/foo.tsx' does not exist.",
+    "File '/node_modules/foo.d.ts' does not exist.",
+    "Found 'package.json' at '/node_modules/foo/package.json'.",
+    "'package.json' has 'types' field 'foo.js' that references '/node_modules/foo/foo.js'.",
+    "File '/node_modules/foo/foo.js' exist - use it as a name resolution result.",
+    "File '/node_modules/foo/foo.js' has an unsupported extension, so skipping it.",
+    "File '/node_modules/foo/foo.js.ts' does not exist.",
+    "File '/node_modules/foo/foo.js.tsx' does not exist.",
+    "File '/node_modules/foo/foo.js.d.ts' does not exist.",
+    "File '/node_modules/foo/index.ts' does not exist.",
+    "File '/node_modules/foo/index.tsx' does not exist.",
+    "File '/node_modules/foo/index.d.ts' does not exist.",
+    "File '/node_modules/@types/foo.d.ts' does not exist.",
+    "File '/node_modules/@types/foo/package.json' does not exist.",
+    "File '/node_modules/@types/foo/index.d.ts' does not exist.",
+    "Loading module 'foo' from 'node_modules' folder.",
+    "File '/node_modules/foo.js' does not exist.",
+    "File '/node_modules/foo.jsx' does not exist.",
+    "Found 'package.json' at '/node_modules/foo/package.json'.",
+    "'package.json' does not have a 'types' or 'main' field.",
+    "File '/node_modules/foo/index.js' does not exist.",
+    "File '/node_modules/foo/index.jsx' does not exist.",
+    "======== Module name 'foo' was not resolved. ========"
+]
\ No newline at end of file
diff --git a/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.types b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.types
new file mode 100644
index 0000000000000..11c9c72cc6907
--- /dev/null
+++ b/tests/baselines/reference/moduleResolutionWithExtensions_unexpected2.types
@@ -0,0 +1,4 @@
+=== /a.ts ===
+import "foo";
+No type information for this code.
+No type information for this code.
\ No newline at end of file
diff --git a/tests/cases/compiler/moduleResolutionWithExtensions_unexpected.ts b/tests/cases/compiler/moduleResolutionWithExtensions_unexpected.ts
new file mode 100644
index 0000000000000..3af02eabb7656
--- /dev/null
+++ b/tests/cases/compiler/moduleResolutionWithExtensions_unexpected.ts
@@ -0,0 +1,12 @@
+// @noImplicitReferences: true
+// @traceResolution: true
+// This tests that a package.json "main" with an unexpected extension is ignored.
+
+// @Filename: /node_modules/normalize.css/normalize.css
+This file is not read.
+
+// @Filename: /node_modules/normalize.css/package.json
+{ "main": "normalize.css" }
+
+// @Filename: /a.ts
+import "normalize.css";
diff --git a/tests/cases/compiler/moduleResolutionWithExtensions_unexpected2.ts b/tests/cases/compiler/moduleResolutionWithExtensions_unexpected2.ts
new file mode 100644
index 0000000000000..1c960fa67e9a6
--- /dev/null
+++ b/tests/cases/compiler/moduleResolutionWithExtensions_unexpected2.ts
@@ -0,0 +1,12 @@
+// @noImplicitReferences: true
+// @traceResolution: true
+// This tests that a package.json "types" with an unexpected extension is ignored.
+
+// @Filename: /node_modules/foo/foo.js
+This file is not read.
+
+// @Filename: /node_modules/foo/package.json
+{ "types": "foo.js" }
+
+// @Filename: /a.ts
+import "foo";