diff --git a/django/contrib/auth/hashers.py b/django/contrib/auth/hashers.py
index b56f74f0d9..0188f4a4e7 100644
--- a/django/contrib/auth/hashers.py
+++ b/django/contrib/auth/hashers.py
@@ -337,6 +337,10 @@ class BCryptSHA256PasswordHasher(BasePasswordHasher):
             (_('checksum'), mask_hash(checksum)),
         ])
 
+    def must_update(self, encoded):
+        algorithm, empty, algostr, rounds, data = encoded.split('$', 4)
+        return int(rounds) != self.rounds
+
 
 class BCryptPasswordHasher(BCryptSHA256PasswordHasher):
     """
diff --git a/docs/releases/1.9.txt b/docs/releases/1.9.txt
index 41ac4f27c9..3bcbd8bbfe 100644
--- a/docs/releases/1.9.txt
+++ b/docs/releases/1.9.txt
@@ -56,6 +56,9 @@ Minor features
   subclassed ``django.contrib.auth.hashers.PBKDF2PasswordHasher`` to change the
   default value.
 
+* The ``BCryptSHA256PasswordHasher`` will now update passwords if its
+  ``rounds`` attribute is changed.
+
 :mod:`django.contrib.gis`
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/tests/auth_tests/test_hashers.py b/tests/auth_tests/test_hashers.py
index f264926aff..993010b2f4 100644
--- a/tests/auth_tests/test_hashers.py
+++ b/tests/auth_tests/test_hashers.py
@@ -177,6 +177,38 @@ class TestUtilsHashPass(SimpleTestCase):
         self.assertTrue(check_password('', blank_encoded))
         self.assertFalse(check_password(' ', blank_encoded))
 
+    @skipUnless(bcrypt, "bcrypt not installed")
+    def test_bcrypt_upgrade(self):
+        hasher = get_hasher('bcrypt')
+        self.assertEqual('bcrypt', hasher.algorithm)
+        self.assertNotEqual(hasher.rounds, 4)
+
+        old_rounds = hasher.rounds
+        try:
+            # Generate a password with 4 rounds.
+            hasher.rounds = 4
+            encoded = make_password('letmein', hasher='bcrypt')
+            rounds = hasher.safe_summary(encoded)['work factor']
+            self.assertEqual(rounds, '04')
+
+            state = {'upgraded': False}
+
+            def setter(password):
+                state['upgraded'] = True
+
+            # Check that no upgrade is triggered.
+            self.assertTrue(check_password('letmein', encoded, setter, 'bcrypt'))
+            self.assertFalse(state['upgraded'])
+
+            # Revert to the old rounds count and ...
+            hasher.rounds = old_rounds
+
+            # ... check if the password would get updated to the new count.
+            self.assertTrue(check_password('letmein', encoded, setter, 'bcrypt'))
+            self.assertTrue(state['upgraded'])
+        finally:
+            hasher.rounds = old_rounds
+
     def test_unusable(self):
         encoded = make_password(None)
         self.assertEqual(len(encoded), len(UNUSABLE_PASSWORD_PREFIX) + UNUSABLE_PASSWORD_SUFFIX_LENGTH)