diff --git a/src/classic/class/ReactClass.js b/src/classic/class/ReactClass.js
index 39a5b73b2f..67a62c1f0b 100644
--- a/src/classic/class/ReactClass.js
+++ b/src/classic/class/ReactClass.js
@@ -828,6 +828,14 @@ var ReactClass = {
       // This constructor is overridden by mocks. The argument is used
       // by mocks to assert on what gets mounted.
 
+      if (__DEV__) {
+        warning(
+          this instanceof Constructor,
+          'Something is calling a React component directly. Use a factory or ' +
+          'JSX instead. See: http://fb.me/react-legacyfactory'
+        );
+      }
+
       // Wire up auto-binding
       if (this.__reactAutoBindMap) {
         bindAutoBindMethods(this);
diff --git a/src/classic/class/__tests__/ReactClass-test.js b/src/classic/class/__tests__/ReactClass-test.js
index eefe02474f..49a0e6b439 100644
--- a/src/classic/class/__tests__/ReactClass-test.js
+++ b/src/classic/class/__tests__/ReactClass-test.js
@@ -363,4 +363,19 @@ describe('ReactClass-spec', function() {
     ).not.toThrow();
   });
 
+  it('should throw when using legacy factories', function() {
+    var Component = React.createClass({
+      render() {
+        return <div />;
+      }
+    });
+
+    expect(() => Component()).toThrow();
+    expect(console.warn.calls.length).toBe(1);
+    expect(console.warn.argsForCall[0][0]).toBe(
+      'Warning: Something is calling a React component directly. Use a ' +
+      'factory or JSX instead. See: http://fb.me/react-legacyfactory'
+    );
+  });
+
 });