diff --git a/package.json b/package.json
index b77589a2..4903865a 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
   "author": "The MessagePack community",
   "license": "ISC",
   "main": "./dist/index.js",
-  "module": "./dist.es5+esm/index.js",
+  "module": "./dist.es5+esm/index.mjs",
   "cdn": "./dist.es5+umd/msgpack.min.js",
   "unpkg": "./dist.es5+umd/msgpack.min.js",
   "types": "./dist/index.d.ts",
diff --git a/tools/esmify.ts b/tools/esmify.ts
index b3a22a4c..f91e18f7 100644
--- a/tools/esmify.ts
+++ b/tools/esmify.ts
@@ -6,8 +6,19 @@ import fs from "fs";
 const files = process.argv.slice(2);
 
 for (const file of files) {
-  console.info(`Processing ${file}`);
-  const content = fs.readFileSync(file).toString("utf8");
-  const newContent = content.replace(/\bfrom "([^"]+)";/g, 'from "$1.js";');
-  fs.writeFileSync(file, newContent);
+  const fileMjs = file.replace(/\.js$/, ".mjs");
+  console.info(`Processing ${file} => ${fileMjs}`);
+  // .js => .mjs
+  const content = fs.readFileSync(file).toString("utf-8");
+  const newContent = content.replace(/\bfrom "([^"]+)";/g, 'from "$1.mjs";')
+    .replace(/\/\/# sourceMappingURL=(.+)\.js\.map$/,
+      "//# sourceMappingURL=$1.mjs.map");
+  fs.writeFileSync(fileMjs, newContent);
+  fs.unlinkSync(file);
+
+  // .js.map => .mjs.map
+  const mapping = JSON.parse(fs.readFileSync(`${file}.map`).toString("utf-8"));
+  mapping.file = mapping.file.replace(/\.js$/, ".mjs");
+  fs.writeFileSync(`${fileMjs}.map`, JSON.stringify(mapping));
+  fs.unlinkSync(`${file}.map`);
 }