diff --git a/package.json b/package.json
index ff75bd6c..8c105f54 100644
--- a/package.json
+++ b/package.json
@@ -25,6 +25,7 @@
     "node": ">=10"
   },
   "main": "lib/index.js",
+  "module": "lib/index.mjs",
   "types": "lib/index.d.ts",
   "browser": "umd/graphql-ws.js",
   "files": [
@@ -42,9 +43,10 @@
     "lint": "eslint 'src'",
     "type-check": "tsc --noEmit",
     "test": "jest",
-    "build:lib": "tsc -b tsconfig.build.json",
+    "build:esm": "tsc -b tsconfig.esm.json && find ./lib -name \"*.js\" -exec sh -c 'mv \"$0\" \"${0%.js}.mjs\"' \"{}\" \\;",
+    "build:cjs": "tsc -b tsconfig.cjs.json",
     "build:umd": "rollup -c && gzip umd/graphql-ws.min.js -c > umd/graphql-ws.min.js.gz",
-    "build": "yarn build:lib && yarn build:umd",
+    "build": "yarn build:esm && yarn build:cjs && yarn build:umd",
     "release": "semantic-release"
   },
   "peerDependencies": {
diff --git a/tsconfig.cjs.json b/tsconfig.cjs.json
new file mode 100644
index 00000000..12694c27
--- /dev/null
+++ b/tsconfig.cjs.json
@@ -0,0 +1,9 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "module": "commonjs",
+    "outDir": "./lib",
+    "declaration": false // already built by `tsconfig.esm.json`
+  },
+  "exclude": ["src/tests", "lib"]
+}
diff --git a/tsconfig.build.json b/tsconfig.esm.json
similarity index 84%
rename from tsconfig.build.json
rename to tsconfig.esm.json
index bf13fd3d..6c152431 100644
--- a/tsconfig.build.json
+++ b/tsconfig.esm.json
@@ -1,7 +1,7 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
-    "module": "commonjs",
+    "module": "es2015",
     "outDir": "./lib",
     "declaration": true
   },