diff --git a/activemodel/lib/active_model/serializers/json.rb b/activemodel/lib/active_model/serializers/json.rb
index 4fbccd7419e01..3c713311fd828 100644
--- a/activemodel/lib/active_model/serializers/json.rb
+++ b/activemodel/lib/active_model/serializers/json.rb
@@ -88,19 +88,17 @@ module JSON
       #                    "title": "So I was thinking"}]}
 
       def as_json(options = nil)
-        hash = serializable_hash(options)
-
-        include_root = include_root_in_json
-        if options.try(:key?, :root)
-          include_root = options[:root]
-        end
-
-        if include_root
-          custom_root = options && options[:root]
-          hash = { custom_root || self.class.model_name.element => hash }
+        opts_root = options[:root] if options.try(:key?, :root)
+        if opts_root
+          custom_root = opts_root == true ? self.class.model_name.element : opts_root
+          { custom_root => serializable_hash(options) }
+        elsif opts_root == false
+          serializable_hash(options)
+        elsif include_root_in_json
+          { self.class.model_name.element => serializable_hash(options) }
+        else
+          serializable_hash(options)
         end
-
-        hash
       end
 
       def from_json(json, include_root=include_root_in_json)
diff --git a/activemodel/test/cases/serializers/json_serialization_test.rb b/activemodel/test/cases/serializers/json_serialization_test.rb
index 5e1e7d897a3e5..40fdcf20cacf2 100644
--- a/activemodel/test/cases/serializers/json_serialization_test.rb
+++ b/activemodel/test/cases/serializers/json_serialization_test.rb
@@ -56,6 +56,16 @@ def setup
     end
   end
 
+  test "should include root in json (option) even if the default is set to false" do
+    begin
+      Contact.include_root_in_json = false
+      json = @contact.to_json(:root => true)
+      assert_match %r{^\{"contact":\{}, json
+    ensure
+      Contact.include_root_in_json = true
+    end
+  end
+
   test "should not include root in json (option)" do
 
     json = @contact.to_json(:root => false)