diff --git a/lib/execjs/runtimes.rb b/lib/execjs/runtimes.rb
index 4bc2489..9216bf4 100644
--- a/lib/execjs/runtimes.rb
+++ b/lib/execjs/runtimes.rb
@@ -17,6 +17,10 @@ module ExecJS
       :command => "node",
       :runner_path => runner_path("node.js")
     )
+
+    Spidermonkey = Runtime.new(
+      :command => "js",
+      :runner_path => runner_path("spidermonkey.js")
     )
 
     V8 = Runtime.new(
diff --git a/lib/execjs/runtimes/spidermonkey.js b/lib/execjs/runtimes/spidermonkey.js
new file mode 100644
index 0000000..f7fbedb
--- /dev/null
+++ b/lib/execjs/runtimes/spidermonkey.js
@@ -0,0 +1,18 @@
+(function(program, execJS) { execJS(program) })(function() { #{source}
+}, function(program) {
+  var output;
+  try {
+    result = program();
+    if (typeof result == 'undefined' && result !== null) {
+      print('["ok"]');
+    } else {
+      try {
+        print(JSON.stringify(['ok', result]));
+      } catch (err) {
+        print('["err"]');
+      }
+    }
+  } catch (err) {
+    print(JSON.stringify(['err', '' + err]));
+  }
+});
diff --git a/test/test_runtimes.rb b/test/test_runtimes.rb
index 7c3d656..7633941 100644
--- a/test/test_runtimes.rb
+++ b/test/test_runtimes.rb
@@ -55,6 +55,14 @@ class TestNodeRuntime < Test::Unit::TestCase
   end
 end
 
+class TestSpidermonkeyRuntime < Test::Unit::TestCase
+  include TestRuntime
+
+  def setup
+    @runtime = ExecJS::Runtimes::Spidermonkey
+  end
+end
+
 class TestV8Runtime < Test::Unit::TestCase
   include TestRuntime