diff --git a/README.md b/README.md
index 809a308..b0a27c6 100644
--- a/README.md
+++ b/README.md
@@ -8,9 +8,11 @@ returns the result to you as a Ruby object.
 ExecJS supports these runtimes:
 
 * [therubyracer](https://github.com/cowboyd/therubyracer) - Google V8
-  embedded within MRI Ruby
+  embedded within Ruby
 * [therubyrhino](https://github.com/cowboyd/therubyrhino) - Mozilla
   Rhino embedded within JRuby
+* [Johnson](https://github.com/jbarnette/johnson) - Mozilla
+  SpiderMonkey embedded within Ruby
 * [Mustang](https://github.com/nu7hatch/mustang) - Mustang V8
   embedded within Ruby
 * [Node.js](http://nodejs.org/)
diff --git a/execjs.gemspec b/execjs.gemspec
index 936de8c..17f3009 100644
--- a/execjs.gemspec
+++ b/execjs.gemspec
@@ -13,6 +13,7 @@ Gem::Specification.new do |s|
   s.files = [
     "lib/execjs.rb",
     "lib/execjs/external_runtime.rb",
+    "lib/execjs/johnson_runtime.rb",
     "lib/execjs/module.rb",
     "lib/execjs/version.rb",
     "lib/execjs/mustang_runtime.rb",
@@ -29,7 +30,11 @@ Gem::Specification.new do |s|
   ]
 
   s.add_dependency "multi_json", "~>1.0"
+  s.add_development_dependency "johnson"
+  s.add_development_dependency "mustang"
   s.add_development_dependency "rake"
+  s.add_development_dependency "therubyracer"
+  s.add_development_dependency "therubyrhino"
 
   s.authors = ["Sam Stephenson", "Josh Peek"]
   s.email   = ["sstephenson@gmail.com", "josh@joshpeek.com"]
diff --git a/lib/execjs/johnson_runtime.rb b/lib/execjs/johnson_runtime.rb
new file mode 100644
index 0000000..1d3e1ff
--- /dev/null
+++ b/lib/execjs/johnson_runtime.rb
@@ -0,0 +1,112 @@
+module ExecJS
+  class JohnsonRuntime
+    class Context
+      def initialize(source = "")
+        @runtime = Johnson::Runtime.new
+        @runtime.evaluate(source)
+      end
+
+      def exec(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
+        if /\S/ =~ source
+          eval "(function(){#{source}})()", options
+        end
+      end
+
+      def eval(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
+        if /\S/ =~ source
+          unbox @runtime.evaluate("(#{source})")
+        end
+      rescue Johnson::Error => e
+        if syntax_error?(e)
+          raise RuntimeError, e.message
+        else
+          raise ProgramError, e.message
+        end
+      end
+
+      def call(properties, *args)
+        unbox @runtime.evaluate(properties).call(*args)
+      rescue Johnson::Error => e
+        if syntax_error?(e)
+          raise RuntimeError, e.message
+        else
+          raise ProgramError, e.message
+        end
+      end
+
+      def unbox(value)
+        case
+        when function?(value)
+          nil
+        when string?(value)
+          value.respond_to?(:force_encoding) ?
+            value.force_encoding('UTF-8') :
+            value
+        when array?(value)
+          value.map { |v| unbox(v) }
+        when object?(value)
+          value.inject({}) do |vs, (k, v)|
+            vs[k] = unbox(v) unless function?(v)
+            vs
+          end
+        else
+          value
+        end
+      end
+
+      private
+        def syntax_error?(error)
+          error.message =~ /^syntax error at /
+        end
+
+        def function?(value)
+          value.respond_to?(:function?) && value.function?
+        end
+
+        def string?(value)
+          value.is_a?(String)
+        end
+
+        def array?(value)
+          array_test.call(value)
+        end
+
+        def object?(value)
+          value.respond_to?(:inject)
+        end
+
+        def array_test
+          @array_test ||= @runtime.evaluate("(function(a) {return a instanceof [].constructor})")
+        end
+    end
+
+    def name
+      "Johnson (SpiderMonkey)"
+    end
+
+    def exec(source)
+      context = Context.new
+      context.exec(source)
+    end
+
+    def eval(source)
+      context = Context.new
+      context.eval(source)
+    end
+
+    def compile(source)
+      Context.new(source)
+    end
+
+    def available?
+      require "johnson"
+      true
+    rescue LoadError
+      false
+    end
+  end
+end
diff --git a/lib/execjs/runtimes.rb b/lib/execjs/runtimes.rb
index 80e11cc..83b92f6 100644
--- a/lib/execjs/runtimes.rb
+++ b/lib/execjs/runtimes.rb
@@ -1,5 +1,6 @@
 require "execjs/module"
 require "execjs/external_runtime"
+require "execjs/johnson_runtime"
 require "execjs/mustang_runtime"
 require "execjs/ruby_racer_runtime"
 require "execjs/ruby_rhino_runtime"
@@ -10,6 +11,8 @@ module ExecJS
 
     RubyRhino = RubyRhinoRuntime.new
 
+    Johnson = JohnsonRuntime.new
+
     Mustang = MustangRuntime.new
 
     Node = ExternalRuntime.new(
@@ -70,6 +73,7 @@ module ExecJS
       @runtimes ||= [
         RubyRacer,
         RubyRhino,
+        Johnson,
         Mustang,
         Node,
         JavaScriptCore,