diff --git a/.github/workflows/nodejs.yml b/.github/workflows/nodejs.yml
index 53e4e28..2aa3b8f 100644
--- a/.github/workflows/nodejs.yml
+++ b/.github/workflows/nodejs.yml
@@ -28,8 +28,12 @@ jobs:
       - name: Install dependencies
         if: steps.yarn-cache.outputs.cache-hit != 'true'
         run: yarn install
-      - name: Build
+      - name: Build (dual CJS + ESM)
         run: yarn build
+      - name: Run CJS integration test
+        run: yarn test:integration:cjs
+      - name: Run ESM integration test
+        run: yarn test:integration:esm
       - name: Run examples
         run: node examples/example.js
 
@@ -54,11 +58,13 @@ jobs:
       - name: Install dependencies
         if: steps.yarn-cache.outputs.cache-hit != 'true'
         run: yarn install
-      - name: Build
+      - name: Build (dual CJS + ESM)
         run: yarn build
       - name: Setup Deno
         uses: denoland/setup-deno@v2
         with:
           deno-version: v2.x
-      - name: Run Deno integration test
+      - name: Run Deno CJS integration test
         run: deno run --allow-read test/integration/deno.ts
+      - name: Run Deno ESM integration test
+        run: deno run --allow-read test/integration/deno-esm.ts
diff --git a/examples/example.js b/examples/example.js
index 4651f7e..0a3c174 100644
--- a/examples/example.js
+++ b/examples/example.js
@@ -1,4 +1,4 @@
-const NtpTimeSync = require("../dist/index").NtpTimeSync;
+const NtpTimeSync = require("../dist/cjs/index").NtpTimeSync;
 
 (async function() {
   const ntpInstance1 = NtpTimeSync.getInstance();
diff --git a/package.json b/package.json
index 098b5bb..944de65 100644
--- a/package.json
+++ b/package.json
@@ -2,8 +2,24 @@
   "name": "ntp-time-sync",
   "version": "0.5.0",
   "description": "Fetches the current time from NTP servers and returns offset information",
-  "main": "dist/index.js",
+  "main": "./dist/cjs/index.js",
+  "module": "./dist/esm/index.js",
+  "types": "./dist/cjs/index.d.ts",
   "type": "commonjs",
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./dist/esm/index.d.ts",
+        "default": "./dist/esm/index.js"
+      },
+      "require": {
+        "types": "./dist/cjs/index.d.ts",
+        "default": "./dist/cjs/index.js"
+      }
+    },
+    "./package.json": "./package.json"
+  },
+  "sideEffects": false,
   "dependencies": {
     "ntp-packet-parser": "^0.6.0"
   },
@@ -15,7 +31,14 @@
   },
   "scripts": {
     "prepublishOnly": "yarn prettier:lint && yarn build",
-    "build": "tsc",
+    "clean": "rm -rf dist",
+    "build:cjs": "tsc -p tsconfig.cjs.json && node scripts/write-package-type.mjs cjs",
+    "build:esm": "tsc -p tsconfig.esm.json && node scripts/write-package-type.mjs esm",
+    "build": "yarn build:cjs && yarn build:esm",
+    "test:integration:cjs": "node test/integration/cjs.cjs",
+    "test:integration:esm": "node test/integration/esm.mjs",
+    "test:integration:deno": "deno run --allow-read test/integration/deno.ts && deno run --allow-read test/integration/deno-esm.ts",
+    "test:integration": "yarn test:integration:cjs && yarn test:integration:esm",
     "prettier": "prettier --write src/**/*.ts",
     "prettier:lint": "prettier --list-different src/**/*.ts"
   },
diff --git a/scripts/write-package-type.mjs b/scripts/write-package-type.mjs
new file mode 100644
index 0000000..d24b210
--- /dev/null
+++ b/scripts/write-package-type.mjs
@@ -0,0 +1,25 @@
+// Writes the appropriate `package.json` marker into dist/{cjs,esm} so Node's
+// module resolver treats the emitted .js files with the correct format.
+//
+// Usage: node scripts/write-package-type.mjs <cjs|esm>
+
+import { mkdirSync, writeFileSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const target = process.argv[2];
+if (target !== "cjs" && target !== "esm") {
+  console.error(`Usage: node scripts/write-package-type.mjs <cjs|esm>`);
+  process.exit(1);
+}
+
+const type = target === "cjs" ? "commonjs" : "module";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+const outPath = resolve(__dirname, "..", "dist", target, "package.json");
+
+mkdirSync(dirname(outPath), { recursive: true });
+writeFileSync(outPath, JSON.stringify({ type }) + "\n");
+
+console.log(`wrote ${outPath} → {"type":"${type}"}`);
diff --git a/src/NtpTimeSync.ts b/src/NtpTimeSync.ts
index 4c075e0..159693c 100644
--- a/src/NtpTimeSync.ts
+++ b/src/NtpTimeSync.ts
@@ -1,9 +1,9 @@
 "use strict";
 
-import * as dgram from "dgram";
+import * as dgram from "node:dgram";
 import { NtpPacket, NtpPacketParser } from "ntp-packet-parser";
-import { NtpTimeResult } from "./NtpTimeResult";
-import { RecursivePartial } from "./RecursivePartial";
+import { NtpTimeResult } from "./NtpTimeResult.js";
+import { RecursivePartial } from "./RecursivePartial.js";
 
 let singleton: NtpTimeSync | undefined;
 let lastPoll: number | undefined;
diff --git a/src/index.ts b/src/index.ts
index 48af6c3..3fe158d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,2 +1,2 @@
-export * from "./NtpTimeResult";
-export * from "./NtpTimeSync";
+export * from "./NtpTimeResult.js";
+export * from "./NtpTimeSync.js";
diff --git a/test/integration/cjs.cjs b/test/integration/cjs.cjs
new file mode 100644
index 0000000..74cdc7b
--- /dev/null
+++ b/test/integration/cjs.cjs
@@ -0,0 +1,45 @@
+// CJS integration test — verifies the dual build's CommonJS output works for plain `require()` consumers.
+// Run: node test/integration/cjs.cjs
+
+"use strict";
+
+const assert = require("node:assert/strict");
+
+const mod = require("../../dist/cjs/index.js");
+const { NtpTimeSync, NtpTimeSyncDefaultOptions } = mod;
+
+assert.equal(typeof NtpTimeSync, "function", "NtpTimeSync should be exported as a class");
+assert.ok(NtpTimeSyncDefaultOptions && typeof NtpTimeSyncDefaultOptions === "object", "defaults should be exported");
+
+// Instantiate with nested options to exercise recursiveResolveOptions → isPlainObject
+const instance = new NtpTimeSync({
+  sampleCount: 4,
+  ntpDefaults: {
+    minPoll: 4,
+    maxPoll: 10,
+  },
+});
+assert.ok(instance instanceof NtpTimeSync, "constructor should return an NtpTimeSync instance");
+
+// Verify singleton
+const a = NtpTimeSync.getInstance();
+const b = NtpTimeSync.getInstance();
+assert.equal(a, b, "getInstance must return the same singleton");
+assert.ok(a instanceof NtpTimeSync, "singleton must be an NtpTimeSync instance");
+
+// Verify API surface (shape only — no network calls)
+assert.equal(typeof a.getTime, "function", "getTime must be a method");
+assert.equal(typeof a.now, "function", "now must be a method");
+assert.equal(typeof a.getNetworkTime, "function", "getNetworkTime must be a method");
+
+// Verify no __proto__ pollution in resolved options
+const opts = /** @type {any} */ (a).options;
+assert.ok(opts, "instance.options must be defined");
+assert.equal(Object.getPrototypeOf(opts.ntpDefaults), Object.prototype, "ntpDefaults must have Object.prototype");
+assert.ok(!Object.prototype.hasOwnProperty.call(opts, "__proto__"), "no __proto__ own property on options");
+assert.ok(
+  !Object.prototype.hasOwnProperty.call(opts.ntpDefaults, "__proto__"),
+  "no __proto__ own property on ntpDefaults"
+);
+
+console.log("ok - cjs integration test passed");
diff --git a/test/integration/deno-esm.ts b/test/integration/deno-esm.ts
new file mode 100644
index 0000000..32eef37
--- /dev/null
+++ b/test/integration/deno-esm.ts
@@ -0,0 +1,36 @@
+// Deno ESM integration test — verifies the ESM dist loads natively under Deno.
+// Run: deno run --allow-read test/integration/deno-esm.ts
+
+import { NtpTimeSync } from "../../dist/esm/index.js";
+
+// Instantiate with nested options to exercise recursiveResolveOptions → isPlainObject
+new NtpTimeSync({
+  sampleCount: 4,
+  ntpDefaults: {
+    minPoll: 4,
+    maxPoll: 10,
+  },
+});
+
+// Verify singleton
+const a = NtpTimeSync.getInstance();
+const b = NtpTimeSync.getInstance();
+if (a !== b) {
+  throw new Error("Singleton check failed: expected same instance");
+}
+
+// Verify no __proto__ pollution in resolved options
+const opts = (a as unknown as { options: { ntpDefaults: Record<string, unknown> } }).options;
+if (Object.getPrototypeOf(opts.ntpDefaults) !== Object.prototype) {
+  throw new Error("ntpDefaults prototype is not Object.prototype");
+}
+if (Object.prototype.hasOwnProperty.call(opts, "__proto__")) {
+  throw new Error("options has unexpected __proto__ own property");
+}
+if (Object.prototype.hasOwnProperty.call(opts.ntpDefaults, "__proto__")) {
+  throw new Error("ntpDefaults has unexpected __proto__ own property");
+}
+
+console.log("✓ NtpTimeSync imported (ESM) and instantiated");
+console.log("✓ Nested options merged without __proto__ issues");
+console.log("✓ Singleton pattern works");
diff --git a/test/integration/deno.ts b/test/integration/deno.ts
index f4b077f..ec54524 100644
--- a/test/integration/deno.ts
+++ b/test/integration/deno.ts
@@ -1,11 +1,11 @@
-// Deno integration test — verifies ntp-time-sync works without --unstable-unsafe-proto
-// Run: deno run test/integration/deno.ts
+// Deno integration test — verifies the CJS dist loads under Deno via createRequire.
+// Run: deno run --allow-read test/integration/deno.ts
 
 import { createRequire } from "node:module";
 
 const require = createRequire(import.meta.url);
 // eslint-disable-next-line @typescript-eslint/no-var-requires
-const { NtpTimeSync } = require("../../dist/index.js");
+const { NtpTimeSync } = require("../../dist/cjs/index.js");
 
 // Instantiate with nested options to exercise recursiveResolveOptions → isPlainObject
 new NtpTimeSync({
@@ -23,6 +23,18 @@ if (a !== b) {
   throw new Error("Singleton check failed: expected same instance");
 }
 
+// Verify no __proto__ pollution in resolved options
+const opts = a.options;
+if (Object.getPrototypeOf(opts.ntpDefaults) !== Object.prototype) {
+  throw new Error("ntpDefaults prototype is not Object.prototype");
+}
+if (Object.prototype.hasOwnProperty.call(opts, "__proto__")) {
+  throw new Error("options has unexpected __proto__ own property");
+}
+if (Object.prototype.hasOwnProperty.call(opts.ntpDefaults, "__proto__")) {
+  throw new Error("ntpDefaults has unexpected __proto__ own property");
+}
+
 console.log("✓ NtpTimeSync imported and instantiated");
 console.log("✓ Nested options merged without __proto__ issues");
 console.log("✓ Singleton pattern works");
diff --git a/test/integration/esm.mjs b/test/integration/esm.mjs
new file mode 100644
index 0000000..9a990d5
--- /dev/null
+++ b/test/integration/esm.mjs
@@ -0,0 +1,42 @@
+// ESM integration test — verifies the dual build's ESM output works for `import` consumers.
+// Run: node test/integration/esm.mjs
+
+import assert from "node:assert/strict";
+
+import { NtpTimeSync, NtpTimeSyncDefaultOptions } from "../../dist/esm/index.js";
+
+assert.equal(typeof NtpTimeSync, "function", "NtpTimeSync should be exported as a class");
+assert.ok(NtpTimeSyncDefaultOptions && typeof NtpTimeSyncDefaultOptions === "object", "defaults should be exported");
+
+// Instantiate with nested options to exercise recursiveResolveOptions → isPlainObject
+const instance = new NtpTimeSync({
+  sampleCount: 4,
+  ntpDefaults: {
+    minPoll: 4,
+    maxPoll: 10,
+  },
+});
+assert.ok(instance instanceof NtpTimeSync, "constructor should return an NtpTimeSync instance");
+
+// Verify singleton
+const a = NtpTimeSync.getInstance();
+const b = NtpTimeSync.getInstance();
+assert.equal(a, b, "getInstance must return the same singleton");
+assert.ok(a instanceof NtpTimeSync, "singleton must be an NtpTimeSync instance");
+
+// Verify API surface (shape only — no network calls)
+assert.equal(typeof a.getTime, "function", "getTime must be a method");
+assert.equal(typeof a.now, "function", "now must be a method");
+assert.equal(typeof a.getNetworkTime, "function", "getNetworkTime must be a method");
+
+// Verify no __proto__ pollution in resolved options
+const opts = /** @type {any} */ (a).options;
+assert.ok(opts, "instance.options must be defined");
+assert.equal(Object.getPrototypeOf(opts.ntpDefaults), Object.prototype, "ntpDefaults must have Object.prototype");
+assert.ok(!Object.prototype.hasOwnProperty.call(opts, "__proto__"), "no __proto__ own property on options");
+assert.ok(
+  !Object.prototype.hasOwnProperty.call(opts.ntpDefaults, "__proto__"),
+  "no __proto__ own property on ntpDefaults"
+);
+
+console.log("ok - esm integration test passed");
diff --git a/tsconfig.base.json b/tsconfig.base.json
new file mode 100644
index 0000000..606c89b
--- /dev/null
+++ b/tsconfig.base.json
@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "rootDir": "./src",
+    "allowJs": true,
+    "lib": [
+      "es2019"
+    ],
+    "target": "es2019",
+    "types": [
+      "node"
+    ],
+    "noEmitOnError": true,
+    "noImplicitAny": true,
+    "noImplicitReturns": true,
+    "declaration": true
+  },
+  "include": [
+    "./src/**/*"
+  ]
+}
diff --git a/tsconfig.cjs.json b/tsconfig.cjs.json
new file mode 100644
index 0000000..52d7cdf
--- /dev/null
+++ b/tsconfig.cjs.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.base.json",
+  "compilerOptions": {
+    "module": "commonjs",
+    "moduleResolution": "bundler",
+    "outDir": "./dist/cjs"
+  }
+}
diff --git a/tsconfig.esm.json b/tsconfig.esm.json
new file mode 100644
index 0000000..17d5a98
--- /dev/null
+++ b/tsconfig.esm.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.base.json",
+  "compilerOptions": {
+    "module": "es2022",
+    "moduleResolution": "bundler",
+    "outDir": "./dist/esm"
+  }
+}
diff --git a/tsconfig.json b/tsconfig.json
index 0a168d3..17d5a98 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,22 +1,8 @@
 {
+  "extends": "./tsconfig.base.json",
   "compilerOptions": {
-    "outDir": "./dist",
-    "rootDir": "./src",
-    "allowJs": true,
-    "lib": [
-      "es2019"
-    ],
-    "module": "commonjs",
-    "target": "es2019",
-    "types": [
-      "node"
-    ],
-    "noEmitOnError": true,
-    "noImplicitAny": true,
-    "noImplicitReturns": true,
-    "declaration": true
-  },
-  "include": [
-    "./src/**/*"
-  ]
+    "module": "es2022",
+    "moduleResolution": "bundler",
+    "outDir": "./dist/esm"
+  }
 }