diff --git a/AUTHORS b/AUTHORS
index f6f32a0db3..4fdfda9a94 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -565,6 +565,7 @@ answer newbie questions, and generally made Django that much better:
     Philip Lindborg <philip.lindborg@gmail.com>
     Philippe Raoult <philippe.raoult@n2nsoft.com>
     phil@produxion.net
+    Piotr Jakimiak <piotr.jakimiak@gmail.com>
     Piotr Lewandowski <piotr.lewandowski@gmail.com>
     plisk
     polpak@yahoo.com
diff --git a/django/contrib/sites/migrations/0002_alter_domain_unique.py b/django/contrib/sites/migrations/0002_alter_domain_unique.py
new file mode 100644
index 0000000000..e7f414fb5c
--- /dev/null
+++ b/django/contrib/sites/migrations/0002_alter_domain_unique.py
@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+import django.contrib.sites.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('sites', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='site',
+            name='domain',
+            field=models.CharField(max_length=100, unique=True, validators=[django.contrib.sites.models._simple_domain_name_validator], verbose_name='domain name'),
+        ),
+    ]
diff --git a/django/contrib/sites/models.py b/django/contrib/sites/models.py
index d7026d0355..6471dca136 100644
--- a/django/contrib/sites/models.py
+++ b/django/contrib/sites/models.py
@@ -73,7 +73,7 @@ class SiteManager(models.Manager):
 class Site(models.Model):
 
     domain = models.CharField(_('domain name'), max_length=100,
-        validators=[_simple_domain_name_validator])
+        validators=[_simple_domain_name_validator], unique=True)
     name = models.CharField(_('display name'), max_length=50)
     objects = SiteManager()
 
diff --git a/docs/ref/contrib/sites.txt b/docs/ref/contrib/sites.txt
index 8c71ca0572..4928c92119 100644
--- a/docs/ref/contrib/sites.txt
+++ b/docs/ref/contrib/sites.txt
@@ -23,6 +23,11 @@ The sites framework is mainly based on a simple model:
 
         The domain name associated with the Web site.
 
+        .. versionchanged:: 1.9
+
+            The ``domain`` field was set to be
+            :attr:`~django.db.models.Field.unique`.
+
     .. attribute:: name
 
         A human-readable "verbose" name for the Web site.
diff --git a/docs/releases/1.9.txt b/docs/releases/1.9.txt
index 473e64f95d..c2ceb5f5db 100644
--- a/docs/releases/1.9.txt
+++ b/docs/releases/1.9.txt
@@ -447,6 +447,9 @@ Miscellaneous
 
 * Support for PostGIS 1.5 has been dropped.
 
+* The ``django.contrib.sites.models.Site.domain`` field was changed to be
+  :attr:`~django.db.models.Field.unique`.
+
 .. _deprecated-features-1.9:
 
 Features deprecated in 1.9
diff --git a/tests/sites_tests/tests.py b/tests/sites_tests/tests.py
index 128420a17b..563ee6a681 100644
--- a/tests/sites_tests/tests.py
+++ b/tests/sites_tests/tests.py
@@ -135,6 +135,12 @@ class SitesFrameworkTests(TestCase):
         clear_site_cache(Site, instance=site, using='default')
         self.assertEqual(models.SITE_CACHE, {})
 
+    def test_unique_domain(self):
+        site = Site(domain=self.site.domain)
+        msg = 'Site with this Domain name already exists.'
+        with self.assertRaisesMessage(ValidationError, msg):
+            site.validate_unique()
+
 
 class JustOtherRouter(object):
     def allow_migrate(self, db, app_label, **hints):