diff --git a/.changeset/little-cups-deliver.md b/.changeset/little-cups-deliver.md
new file mode 100644
index 0000000000..40da7e8ed1
--- /dev/null
+++ b/.changeset/little-cups-deliver.md
@@ -0,0 +1,5 @@
+---
+"@react-router/dev": patch
+---
+
+When executing `react-router.config.ts` and `routes.ts` with `vite-node`, ensure that PostCSS config files are ignored
diff --git a/packages/react-router-dev/vite/vite-node.ts b/packages/react-router-dev/vite/vite-node.ts
index ed2adb872c..35ce0ebb61 100644
--- a/packages/react-router-dev/vite/vite-node.ts
+++ b/packages/react-router-dev/vite/vite-node.ts
@@ -36,6 +36,15 @@ export async function createContext({
     optimizeDeps: {
       noDiscovery: true,
     },
+    css: {
+      // This empty PostCSS config object prevents the PostCSS config file from
+      // being loaded. We don't need it in a React Router config context, and
+      // there's also an issue in Vite 5 when using a .ts PostCSS config file in
+      // an ESM project: https://github.com/vitejs/vite/issues/15869. Consumers
+      // can work around this in their own Vite config file, but they can't
+      // configure this internal usage of vite-node.
+      postcss: {},
+    },
     configFile: false,
     envFile: false,
     plugins: [],