diff --git a/packages/vite/src/node/build.ts b/packages/vite/src/node/build.ts
index 3091de5ed980ad..25b752d7512df6 100644
--- a/packages/vite/src/node/build.ts
+++ b/packages/vite/src/node/build.ts
@@ -438,21 +438,26 @@ async function doBuild(
   try {
     const buildOutputOptions = (output: OutputOptions = {}): OutputOptions => {
       const cjsSsrBuild = ssr && config.ssr?.format === 'cjs'
+      const format = output.format || (cjsSsrBuild ? 'cjs' : 'es')
+      const jsExt =
+        (ssr && config.ssr?.target !== 'webworker') || libOptions
+          ? resolveOutputJsExtension(format, getPkgJson(config.root)?.type)
+          : 'js'
       return {
         dir: outDir,
         // Default format is 'es' for regular and for SSR builds
-        format: cjsSsrBuild ? 'cjs' : 'es',
+        format,
         exports: cjsSsrBuild ? 'named' : 'auto',
         sourcemap: options.sourcemap,
         name: libOptions ? libOptions.name : undefined,
         generatedCode: 'es2015',
         entryFileNames: ssr
-          ? `[name].js`
+          ? `[name].${jsExt}`
           : libOptions
-          ? resolveLibFilename(libOptions, output.format || 'es', config.root)
+          ? resolveLibFilename(libOptions, format, config.root, jsExt)
           : path.posix.join(options.assetsDir, `[name].[hash].js`),
         chunkFileNames: libOptions
-          ? `[name].[hash].js`
+          ? `[name].[hash].${jsExt}`
           : path.posix.join(options.assetsDir, `[name].[hash].js`),
         assetFileNames: libOptions
           ? `[name].[ext]`
@@ -591,10 +596,24 @@ function getPkgName(name: string) {
   return name?.startsWith('@') ? name.split('/')[1] : name
 }
 
+type JsExt = 'js' | 'cjs' | 'mjs'
+
+function resolveOutputJsExtension(
+  format: ModuleFormat,
+  type: string = 'commonjs'
+): JsExt {
+  if (type === 'module') {
+    return format === 'cjs' || format === 'umd' ? 'cjs' : 'js'
+  } else {
+    return format === 'es' ? 'mjs' : 'js'
+  }
+}
+
 export function resolveLibFilename(
   libOptions: LibraryOptions,
   format: ModuleFormat,
-  root: string
+  root: string,
+  extension?: JsExt
 ): string {
   if (typeof libOptions.fileName === 'function') {
     return libOptions.fileName(format)
@@ -608,13 +627,7 @@ export function resolveLibFilename(
       'Name in package.json is required if option "build.lib.fileName" is not provided.'
     )
 
-  let extension: string
-
-  if (packageJson?.type === 'module') {
-    extension = format === 'cjs' || format === 'umd' ? 'cjs' : 'js'
-  } else {
-    extension = format === 'es' ? 'mjs' : 'js'
-  }
+  extension ??= resolveOutputJsExtension(format, packageJson.type)
 
   if (format === 'cjs' || format === 'es') {
     return `${name}.${extension}`
diff --git a/playground/legacy/__tests__/ssr/serve.ts b/playground/legacy/__tests__/ssr/serve.ts
index 339b95e690b83a..81669a4891b8ef 100644
--- a/playground/legacy/__tests__/ssr/serve.ts
+++ b/playground/legacy/__tests__/ssr/serve.ts
@@ -22,7 +22,7 @@ export async function serve(): Promise<{ close(): Promise<void> }> {
 
   app.use('/', async (_req, res) => {
     const { render } = await import(
-      path.resolve(rootDir, './dist/server/entry-server.js')
+      path.resolve(rootDir, './dist/server/entry-server.mjs')
     )
     const html = await render()
     res.status(200).set({ 'Content-Type': 'text/html' }).end(html)
diff --git a/playground/lib/__tests__/lib.spec.ts b/playground/lib/__tests__/lib.spec.ts
index f8eea2b41847b8..32c6edba1bdeb9 100644
--- a/playground/lib/__tests__/lib.spec.ts
+++ b/playground/lib/__tests__/lib.spec.ts
@@ -40,13 +40,13 @@ describe.runIf(isBuild)('build', () => {
       'hello vite'
     )
     const code = fs.readFileSync(
-      path.join(testDir, 'dist/lib/dynamic-import-message.es.js'),
+      path.join(testDir, 'dist/lib/dynamic-import-message.es.mjs'),
       'utf-8'
     )
     expect(code).not.toMatch('__vitePreload')
 
     // Test that library chunks are hashed
-    expect(code).toMatch(/await import\("\.\/message.[a-z\d]{8}.js"\)/)
+    expect(code).toMatch(/await import\("\.\/message.[a-z\d]{8}.mjs"\)/)
   })
 
   test('@import hoist', async () => {
diff --git a/playground/lib/vite.dyimport.config.js b/playground/lib/vite.dyimport.config.js
index 1f64018c5850cf..54a84dbe76c24a 100644
--- a/playground/lib/vite.dyimport.config.js
+++ b/playground/lib/vite.dyimport.config.js
@@ -10,7 +10,7 @@ module.exports = {
       entry: path.resolve(__dirname, 'src/main2.js'),
       formats: ['es', 'iife'],
       name: 'message',
-      fileName: (format) => `dynamic-import-message.${format}.js`
+      fileName: (format) => `dynamic-import-message.${format}.mjs`
     },
     outDir: 'dist/lib'
   }