Index: Lib/abc.py
===================================================================
--- Lib/abc.py	(revision 87870)
+++ Lib/abc.py	(working copy)
@@ -133,7 +133,7 @@
         return cls
 
     def register(cls, subclass):
-        """Register a virtual subclass of an ABC."""
+        """Register a virtual subclass of an ABC. Returns the said subclass."""
         if not isinstance(subclass, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):
@@ -145,6 +145,7 @@
             raise RuntimeError("Refusing to create an inheritance cycle")
         cls._abc_registry.add(subclass)
         ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
+        return subclass # For usage as a decorator
 
     def _dump_registry(cls, file=None):
         """Debug helper to print the ABC registry."""
Index: Lib/test/test_abc.py
===================================================================
--- Lib/test/test_abc.py	(revision 87870)
+++ Lib/test/test_abc.py	(working copy)
@@ -128,11 +128,12 @@
         self.assertFalse(issubclass(B, (A,)))
         self.assertNotIsInstance(b, A)
         self.assertNotIsInstance(b, (A,))
-        A.register(B)
+        B1 = A.register(B)
         self.assertTrue(issubclass(B, A))
         self.assertTrue(issubclass(B, (A,)))
         self.assertIsInstance(b, A)
         self.assertIsInstance(b, (A,))
+        self.assertIs(B1, B)
         class C(B):
             pass
         c = C()
@@ -140,6 +141,14 @@
         self.assertTrue(issubclass(C, (A,)))
         self.assertIsInstance(c, A)
         self.assertIsInstance(c, (A,))
+        @A.register
+        class D(object):
+            pass
+        d = D()
+        self.assertTrue(issubclass(D, A))
+        self.assertTrue(issubclass(D, (A,)))
+        self.assertIsInstance(d, A)
+        self.assertIsInstance(d, (A,))
 
     def test_isinstance_invalidation(self):
         class A(metaclass=abc.ABCMeta):