diff --git a/packages/playground/legacy/__tests__/legacy.spec.ts b/packages/playground/legacy/__tests__/legacy.spec.ts
index 787f2168cfc8ec..7263174e84b074 100644
--- a/packages/playground/legacy/__tests__/legacy.spec.ts
+++ b/packages/playground/legacy/__tests__/legacy.spec.ts
@@ -1,4 +1,4 @@
-import { isBuild } from '../../testUtils'
+import { isBuild, untilUpdated } from '../../testUtils'
 
 test('should work', async () => {
   expect(await page.textContent('#app')).toMatch('Hello')
@@ -18,3 +18,29 @@ test('wraps with iife', async () => {
     'exposed babel helpers: false'
   )
 })
+
+test('generates assets', async () => {
+  await untilUpdated(
+    () => page.textContent('#assets'),
+    isBuild
+      ? [
+          'index: 404',
+          'index-legacy: 404',
+          'chunk-async: 404',
+          'chunk-async-legacy: 404',
+          'immutable-chunk: 200',
+          'immutable-chunk-legacy: 200',
+          'polyfills-legacy: 404'
+        ].join('\n')
+      : [
+          'index: 404',
+          'index-legacy: 404',
+          'chunk-async: 404',
+          'chunk-async-legacy: 404',
+          'immutable-chunk: 404',
+          'immutable-chunk-legacy: 404',
+          'polyfills-legacy: 404'
+        ].join('\n'),
+    true
+  )
+})
diff --git a/packages/playground/legacy/immutable-chunk.js b/packages/playground/legacy/immutable-chunk.js
new file mode 100644
index 00000000000000..4227b718b98309
--- /dev/null
+++ b/packages/playground/legacy/immutable-chunk.js
@@ -0,0 +1,18 @@
+const chunks = [
+  'index',
+  'index-legacy',
+  'chunk-async',
+  'chunk-async-legacy',
+  'immutable-chunk',
+  'immutable-chunk-legacy',
+  'polyfills-legacy'
+]
+
+export function fn() {
+  return Promise.all(
+    chunks.map(async (name) => {
+      const response = await fetch(`/assets/${name}.js`)
+      return `${name}: ${response.status}`
+    })
+  )
+}
diff --git a/packages/playground/legacy/index.html b/packages/playground/legacy/index.html
index 0dcb9a6c4b2665..bdc2feac6b4fbe 100644
--- a/packages/playground/legacy/index.html
+++ b/packages/playground/legacy/index.html
@@ -2,4 +2,5 @@ <h1 id="app"></h1>
 <div id="env"></div>
 <div id="iterators"></div>
 <div id="babel-helpers"></div>
+<div id="assets"></div>
 <script type="module" src="./main.js"></script>
diff --git a/packages/playground/legacy/main.js b/packages/playground/legacy/main.js
index 78322d69455af0..dc458c199b8754 100644
--- a/packages/playground/legacy/main.js
+++ b/packages/playground/legacy/main.js
@@ -27,6 +27,13 @@ text(
   String.raw`exposed babel helpers: ${window._templateObject != null}`
 )
 
+// dynamic chunk names
+import('./immutable-chunk.js')
+  .then(({ fn }) => fn())
+  .then((assets) => {
+    text('#assets', assets.join('\n'))
+  })
+
 function text(el, text) {
   document.querySelector(el).textContent = text
 }
diff --git a/packages/playground/legacy/vite.config.js b/packages/playground/legacy/vite.config.js
index d4a5e4bd481b84..a8fc762cd501a4 100644
--- a/packages/playground/legacy/vite.config.js
+++ b/packages/playground/legacy/vite.config.js
@@ -11,7 +11,19 @@ module.exports = {
 
   build: {
     // make tests faster
-    minify: false
+    minify: false,
+
+    rollupOptions: {
+      output: {
+        chunkFileNames(chunkInfo) {
+          if (chunkInfo.name === 'immutable-chunk') {
+            return `assets/${chunkInfo.name}.js`
+          }
+
+          return `assets/chunk-[name].[hash].js`
+        }
+      }
+    }
   },
 
   // special test only hook
diff --git a/packages/plugin-legacy/index.js b/packages/plugin-legacy/index.js
index d2ae6fb8f9d809..44d1bbc4e00ebc 100644
--- a/packages/plugin-legacy/index.js
+++ b/packages/plugin-legacy/index.js
@@ -99,7 +99,7 @@ function viteLegacyPlugin(options = {}) {
     },
 
     async generateBundle(opts, bundle) {
-      if (!isLegacyOutput(opts)) {
+      if (!isLegacyBundle(bundle, opts)) {
         if (!modernPolyfills.size) {
           return
         }
@@ -168,8 +168,9 @@ function viteLegacyPlugin(options = {}) {
       }
 
       /**
-       * @param {string|((chunkInfo: import('rollup').PreRenderedChunk)=>string)} fileNames
+       * @param {string | ((chunkInfo: import('rollup').PreRenderedChunk) => string)} fileNames
        * @param {string?} defaultFileName
+       * @returns {(chunkInfo: import('rollup').PreRenderedChunk) => string)}
        */
       const getLegacyOutputFileName = (
         fileNames,
@@ -179,21 +180,20 @@ function viteLegacyPlugin(options = {}) {
           return path.posix.join(config.build.assetsDir, defaultFileName)
         }
 
-        // does not support custom functions.
-        if (typeof fileNames === 'function') {
-          throw new Error(
-            `@vitejs/plugin-legacy rollupOptions.output.entryFileNames and rollupOptions.output.chunkFileNames` +
-              ` does not support the function format.`
-          )
-        }
+        return (chunkInfo) => {
+          let fileName =
+            typeof fileNames === 'function' ? fileNames(chunkInfo) : fileNames
 
-        let fileName = defaultFileName
-        // Custom string file return format.
-        if (fileNames && typeof fileNames === 'string') {
-          fileName = fileNames.replace(/\[name\]/, '[name]-legacy')
-        }
+          if (fileName.includes('[name]')) {
+            // [name]-[hash].[format] -> [name]-legacy-[hash].[format]
+            fileName = fileName.replace('[name]', '[name]-legacy')
+          } else {
+            // entry.js -> entry-legacy.js
+            fileName = fileName.replace(/(.+)\.(.+)/, '$1-legacy.$2')
+          }
 
-        return fileName
+          return fileName
+        }
       }
 
       /**
@@ -219,7 +219,7 @@ function viteLegacyPlugin(options = {}) {
     },
 
     renderChunk(raw, chunk, opts) {
-      if (!isLegacyOutput(opts)) {
+      if (!isLegacyChunk(chunk, opts)) {
         if (
           options.modernPolyfills &&
           !Array.isArray(options.modernPolyfills)
@@ -399,7 +399,7 @@ function viteLegacyPlugin(options = {}) {
     },
 
     generateBundle(opts, bundle) {
-      if (isLegacyOutput(opts)) {
+      if (isLegacyBundle(bundle, opts)) {
         // avoid emitting duplicate assets
         for (const name in bundle) {
           if (bundle[name].type === 'asset') {
@@ -564,14 +564,27 @@ function polyfillsPlugin(imports) {
 }
 
 /**
+ * @param {import('rollup').RenderedChunk} chunk
  * @param {import('rollup').NormalizedOutputOptions} options
  */
-function isLegacyOutput(options) {
-  return (
-    options.format === 'system' &&
-    typeof options.entryFileNames === 'string' &&
-    options.entryFileNames.includes('-legacy')
-  )
+function isLegacyChunk(chunk, options) {
+  return options.format === 'system' && chunk.fileName.includes('-legacy')
+}
+
+/**
+ * @param {import('rollup').OutputBundle} bundle
+ * @param {import('rollup').NormalizedOutputOptions} options
+ */
+function isLegacyBundle(bundle, options) {
+  if (options.format === 'system') {
+    const entryChunk = Object.values(bundle).find(
+      (output) => output.type === 'chunk' && output.isEntry
+    )
+
+    return !!entryChunk && entryChunk.fileName.includes('-legacy')
+  }
+
+  return false
 }
 
 /**