diff --git a/.changeset/clever-tools-enjoy.md b/.changeset/clever-tools-enjoy.md
new file mode 100644
index 000000000000..b6066258a773
--- /dev/null
+++ b/.changeset/clever-tools-enjoy.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+app.js -> app.cjs
diff --git a/packages/kit/src/api/adapt/prerender.js b/packages/kit/src/api/adapt/prerender.js
index ac2f0be76157..2e6b2bef5647 100644
--- a/packages/kit/src/api/adapt/prerender.js
+++ b/packages/kit/src/api/adapt/prerender.js
@@ -47,7 +47,7 @@ export async function prerender({ dir, out, log, config, force }) {
 	const seen = new Set();
 
 	const server_root = resolve_path(dir);
-	const app = require(`${server_root}/server/app.js`);
+	const app = require(`${server_root}/server/app.cjs`);
 
 	app.init({
 		paths: config.paths
diff --git a/packages/kit/src/api/build/index.js b/packages/kit/src/api/build/index.js
index 08334c8aab82..10f218a0a70c 100644
--- a/packages/kit/src/api/build/index.js
+++ b/packages/kit/src/api/build/index.js
@@ -380,9 +380,9 @@ export async function build(config) {
 		dir: `${OPTIMIZED}/server`,
 		format: 'cjs', // TODO some adapters might want ESM?
 		exports: 'named',
-		entryFileNames: '[name].js',
-		chunkFileNames: 'chunks/[name].js',
-		assetFileNames: 'assets/[name].js',
+		entryFileNames: '[name].cjs',
+		chunkFileNames: 'chunks/[name].cjs',
+		assetFileNames: 'assets/[name].cjs',
 		sourcemap: true
 	});
 
diff --git a/packages/kit/src/api/start/index.js b/packages/kit/src/api/start/index.js
index af35cdf17aa0..4219c258cd43 100644
--- a/packages/kit/src/api/start/index.js
+++ b/packages/kit/src/api/start/index.js
@@ -14,7 +14,7 @@ const mutable = (dir) =>
 
 export function start({ port, config }) {
 	return new Promise((fulfil) => {
-		const app = relative('./.svelte/build/optimized/server/app.js');
+		const app = relative('./.svelte/build/optimized/server/app.cjs');
 
 		const static_handler = fs.existsSync(config.files.assets)
 			? mutable(config.files.assets)