diff --git a/src/core/ReactCompositeComponent.js b/src/core/ReactCompositeComponent.js
index 87a8de23b3f..69486ca1e37 100644
--- a/src/core/ReactCompositeComponent.js
+++ b/src/core/ReactCompositeComponent.js
@@ -987,7 +987,19 @@ var ReactCompositeComponentMixin = {
     // TODO: Stop validating prop types here and only use the descriptor
     // validation.
     var componentName = this.constructor.displayName;
-    for (var propName in propTypes) {
+    var propName;
+
+    for (propName in props) {
+      if (props.hasOwnProperty(propName) &&
+          !propTypes.hasOwnProperty(propName)) {
+        var message = location.substring(0,1).toUpperCase() +
+            location.substring(1) + " `" + propName +
+            "` was not expected in " + ("`" + componentName + "`.");
+        warning(false, message);
+      }
+    }
+
+    for (propName in propTypes) {
       if (propTypes.hasOwnProperty(propName)) {
         var error =
           propTypes[propName](props, propName, componentName, location);
diff --git a/src/core/ReactDescriptorValidator.js b/src/core/ReactDescriptorValidator.js
index 4123160d170..1b2dce86017 100644
--- a/src/core/ReactDescriptorValidator.js
+++ b/src/core/ReactDescriptorValidator.js
@@ -199,7 +199,22 @@ function validateChildKeys(component, parentType) {
  * @private
  */
 function checkPropTypes(componentName, propTypes, props, location) {
-  for (var propName in propTypes) {
+  var propName;
+  for (propName in props) {
+    if (props.hasOwnProperty(propName) &&
+        !propTypes.hasOwnProperty(propName)) {
+      var message = location.substring(0,1).toUpperCase() +
+          location.substring(1) + " `" + propName +
+          "` was not expected in " + ("`" + componentName + "`.");
+      loggedTypeFailures[message] = true;
+      monitorCodeUse(
+        'react_failed_descriptor_type_check',
+        { message: message }
+      );
+    }
+  }
+
+  for (propName in propTypes) {
     if (propTypes.hasOwnProperty(propName)) {
       var error;
       // Prop type validation may throw. In case they do, we don't want to
diff --git a/src/core/__tests__/ReactCompositeComponent-test.js b/src/core/__tests__/ReactCompositeComponent-test.js
index a970b171693..e3f1b77197c 100644
--- a/src/core/__tests__/ReactCompositeComponent-test.js
+++ b/src/core/__tests__/ReactCompositeComponent-test.js
@@ -553,6 +553,22 @@ describe('ReactCompositeComponent', function() {
     expect(console.warn.mock.calls.length).toBe(2);
   });
 
+  it('should warn about unexpected props', function() {
+    var Component = React.createClass({
+      propTypes: {},
+      render: function() {
+        return <span>{this.props.prop}</span>;
+      }
+    });
+
+    ReactTestUtils.renderIntoDocument(<Component prop={42} />);
+
+    expect(console.warn.mock.calls.length).toBe(1);
+    expect(console.warn.mock.calls[0][0]).toBe(
+      'Warning: Prop `prop` was not expected in `Component`.'
+    );
+  });
+
   it('should throw on invalid prop types', function() {
     expect(function() {
       React.createClass({