diff --git a/src/__tests__/utils.js b/src/__tests__/utils.js
index c658fbc0..f07d5c91 100644
--- a/src/__tests__/utils.js
+++ b/src/__tests__/utils.js
@@ -190,3 +190,17 @@ function mockCosmiconfig(result = null) {
 
   cosmiconfigSync.mockImplementationOnce(() => ({search: () => result}))
 }
+
+test.each([
+  {format: 'cjs', extension: '.cjs'},
+  {format: 'esm', extension: '.mjs'},
+  {format: 'umd', extension: '.js'},
+  {format: 'amd', extension: '.js'},
+])(
+  'file extension in rollupOutput with $format should be $extension',
+  ({format, extension}) => {
+    expect(
+      require('../utils').getRollupOutput(format).filename.endsWith(extension),
+    ).toBeTruthy()
+  },
+)
diff --git a/src/utils.js b/src/utils.js
index 0f761e57..a515cead 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -210,12 +210,17 @@ function getRollupInputs() {
 function getRollupOutput(format = process.env.BUILD_FORMAT) {
   const minify = parseEnv('BUILD_MINIFY', false)
   const filenameSuffix = process.env.BUILD_FILENAME_SUFFIX || ''
+  const ext =
+    {
+      esm: '.mjs',
+      cjs: '.cjs',
+    }[format] || '.js'
   const filename = [
     pkg.name,
     filenameSuffix,
     `.${format}`,
     minify ? '.min' : null,
-    '.js',
+    ext,
   ]
     .filter(Boolean)
     .join('')