diff --git a/src/Parse/ParseUser.php b/src/Parse/ParseUser.php
index a59433dc..131d1b77 100644
--- a/src/Parse/ParseUser.php
+++ b/src/Parse/ParseUser.php
@@ -157,6 +157,33 @@ public static function logIn($username, $password)
         return $user;
     }
 
+    /**
+     * Uses the master key to log in and return a valid ParseUser, or throws if invalid.
+     *
+     * @param $userId
+     *
+     * @throws ParseException
+     *
+     * @return ParseUser
+     */
+    public static function logInAs($userId)
+    {
+        if (!$userId) {
+            throw new ParseException(
+                'Cannot log in as user with an empty user id',
+                200
+            );
+        }
+        $data = ['userId' => $userId];
+        $result = ParseClient::_request('POST', 'loginAs', '', json_encode($data), true);
+        $user = new static();
+        $user->_mergeAfterFetch($result);
+        $user->handleSaveResult(true);
+        ParseClient::getStorage()->set('user', $user);
+
+        return $user;
+    }
+
     /**
      * Logs in with Facebook details, or throws if invalid.
      *
diff --git a/tests/Parse/ParseUserTest.php b/tests/Parse/ParseUserTest.php
index adf626cd..2e99f912 100644
--- a/tests/Parse/ParseUserTest.php
+++ b/tests/Parse/ParseUserTest.php
@@ -83,6 +83,33 @@ public function testLoginWrongPassword()
         ParseUser::logIn('asdf', 'bogus');
     }
 
+    public function testLoginAsSuccess()
+    {
+        $user = new ParseUser();
+        $user->setUsername('plainusername');
+        $user->setPassword('plainpassword');
+        $user->signUp();
+
+        $id = $user->getObjectId();
+        $loggedInUser = ParseUser::logInAs($id);
+        $this->assertTrue($loggedInUser->isAuthenticated());
+        $this->assertEquals('plainusername', $loggedInUser->get('username'));
+
+        ParseUser::logOut();
+    }
+
+    public function testLoginAsEmptyUsername()
+    {
+        $this->expectException('Parse\ParseException', 'Cannot log in as user with an empty user id.');
+        ParseUser::logInAs('');
+    }
+
+    public function testLoginAsNonexistentUser()
+    {
+        $this->expectException('Parse\ParseException', 'user not found.');
+        ParseUser::logInAs('a1b2c3d4e5');
+    }
+
     public function testLoginWithFacebook()
     {
         $this->expectException(