diff --git a/packages/shared/forks/object-assign.inline-umd.js b/packages/shared/forks/object-assign.inline-umd.js
new file mode 100644
index 0000000000000..12dffb76c8fe6
--- /dev/null
+++ b/packages/shared/forks/object-assign.inline-umd.js
@@ -0,0 +1,31 @@
+/**
+ * Copyright 2004-present Facebook. All Rights Reserved.
+ */
+
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
+const _assign = function(to, from) {
+  for (let key in from) {
+    if (hasOwnProperty.call(from, key)) {
+      to[key] = from[key];
+    }
+  }
+};
+
+export default Object.assign ||
+  function(target, sources) {
+    if (target == null) {
+      throw new TypeError('Object.assign target cannot be null or undefined');
+    }
+
+    const to = Object(target);
+
+    for (let nextIndex = 1; nextIndex < arguments.length; nextIndex++) {
+      const nextSource = arguments[nextIndex];
+      if (nextSource != null) {
+        _assign(to, Object(nextSource));
+      }
+    }
+
+    return to;
+  };
diff --git a/scripts/babel/transform-object-assign-require.js b/scripts/babel/transform-object-assign-require.js
index 79c7421c5e2ef..8b43be53e700c 100644
--- a/scripts/babel/transform-object-assign-require.js
+++ b/scripts/babel/transform-object-assign-require.js
@@ -28,6 +28,10 @@ module.exports = function autoImporter(babel) {
 
     visitor: {
       CallExpression: function(path, file) {
+        if (file.filename.indexOf('object-assign') !== -1) {
+          // Don't replace Object.assign if we're transforming object-assign
+          return;
+        }
         if (path.get('callee').matchesPattern('Object.assign')) {
           // generate identifier and require if it hasn't been already
           const id = getAssignIdent(path, file, this);
@@ -36,6 +40,10 @@ module.exports = function autoImporter(babel) {
       },
 
       MemberExpression: function(path, file) {
+        if (file.filename.indexOf('object-assign') !== -1) {
+          // Don't replace Object.assign if we're transforming object-assign
+          return;
+        }
         if (path.matchesPattern('Object.assign')) {
           const id = getAssignIdent(path, file, this);
           path.replaceWith(id);
diff --git a/scripts/rollup/forks.js b/scripts/rollup/forks.js
index aa0938b7d539d..1fbbd0ffed8dc 100644
--- a/scripts/rollup/forks.js
+++ b/scripts/rollup/forks.js
@@ -22,8 +22,8 @@ const {RENDERER, RECONCILER} = moduleTypes;
 // If you need to replace a file with another file for a specific environment,
 // add it to this list with the logic for choosing the right replacement.
 const forks = Object.freeze({
-  // Optimization: for UMDs, use object-assign polyfill that is already a part
-  // of the React package instead of bundling it again.
+  // Optimization: for UMDs, use a version that we can inline into the React bundle.
+  // Use that from all other bundles.
   'object-assign': (bundleType, entry, dependencies) => {
     if (
       bundleType !== UMD_DEV &&
@@ -34,12 +34,16 @@ const forks = Object.freeze({
       // happens. Other bundles just require('object-assign') anyway.
       return null;
     }
+    if (entry === 'react') {
+      // Use the forked version that uses ES modules instead of CommonJS.
+      return 'shared/forks/object-assign.inline-umd.js';
+    }
     if (dependencies.indexOf('react') === -1) {
       // We can only apply the optimizations to bundle that depend on React
       // because we read assign() from an object exposed on React internals.
       return null;
     }
-    // We can use the fork!
+    // We can use the fork that reads the secret export!
     return 'shared/forks/object-assign.umd.js';
   },