diff --git a/tests/model_package/tests.py b/tests/model_package/tests.py
index 2ac617f880..84b1dd1c30 100644
--- a/tests/model_package/tests.py
+++ b/tests/model_package/tests.py
@@ -15,6 +15,14 @@ class Advertisement(models.Model):
 
 
 class ModelPackageTests(TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        super(ModelPackageTests, cls).setUpClass()
+        # This cleanup is necessary because contrib.sites cache
+        # makes tests interfere with each other, see #11505
+        Site.objects.clear_cache()
+
     def test_m2m_tables_in_subpackage_models(self):
         """
         Regression for #12168: models split into subpackages still get M2M
diff --git a/tests/syndication_tests/tests.py b/tests/syndication_tests/tests.py
index 8da3d5e8e0..63638fa589 100644
--- a/tests/syndication_tests/tests.py
+++ b/tests/syndication_tests/tests.py
@@ -8,6 +8,7 @@ try:
 except ImportError:
     pytz = None
 
+from django.contrib.sites.models import Site
 from django.contrib.syndication import views
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase, override_settings
@@ -55,6 +56,12 @@ class SyndicationFeedTest(FeedTestCase):
     """
     Tests for the high-level syndication feed framework.
     """
+    @classmethod
+    def setUpClass(cls):
+        super(SyndicationFeedTest, cls).setUpClass()
+        # This cleanup is necessary because contrib.sites cache
+        # makes tests interfere with each other, see #11505
+        Site.objects.clear_cache()
 
     def test_rss2_feed(self):
         """