diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index ef83ff093..e6143b17b 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -283,6 +283,7 @@ All definitions files include a header with the author and editors, so at some p
 * [object-path](https://github.com/mariocasciaro/object-path) (by [Paulo Cesar](https://github.com/pocesar/))
 * [ocLazyLoad](https://github.com/ocombe/ocLazyLoad) (by [Roland Zwaga](https://github.com/rolandzwaga/))
 * [OpenLayers](https://github.com/openlayers/openlayers) (by [Ilya Bolkhovsky](https://github.com/bolhovsky/))
+* [opn](https://github.com/sindresorhus/opn) (by [Shinnosuke Watanabe](https://github.com/shinnn))
 * [Optimist](https://github.com/substack/node-optimist) (by [Carlos Ballesteros Velasco](https://github.com/soywiz))
 * [Passport](http://passportjs.org/) (by [Hiroki Horiuchi](https://github.com/horiuchi/))
 * [passport-facebook](https://github.com/jaredhanson/passport-facebook) (by [James Roland Cabresos](https://github.com/staticfunction/))
diff --git a/opn/opn-tests.ts b/opn/opn-tests.ts
new file mode 100644
index 000000000..361725970
--- /dev/null
+++ b/opn/opn-tests.ts
@@ -0,0 +1,10 @@
+/// <reference path="opn.d.ts" />
+
+import opn = require('opn');
+
+var errorCallback: (err: Error) => void;
+
+opn('foo');
+opn('foo', 'bar');
+opn('foo', errorCallback);
+opn('foo', 'bar', errorCallback);
diff --git a/opn/opn.d.ts b/opn/opn.d.ts
new file mode 100644
index 000000000..10a34ce60
--- /dev/null
+++ b/opn/opn.d.ts
@@ -0,0 +1,10 @@
+// Type definitions for opn 1.0.0
+// Project: https://github.com/sindresorhus/opn
+// Definitions by: Shinnosuke Watanabe <https://github.com/shinnn>
+// Definitions: https://github.com/borisyankov/DefinitelyTyped
+
+declare module 'opn' {
+	function opn(target: string, callback?: (err: Error) => void): void;
+	function opn(target: string, app: string, callback?: (err: Error) => void): void;
+	export = opn;
+}