diff --git a/django/test/testcases.py b/django/test/testcases.py
index 010850d42ae..ee22ac219eb 100644
--- a/django/test/testcases.py
+++ b/django/test/testcases.py
@@ -500,6 +500,13 @@ class TransactionTestCase(SimpleTestCase):
         If ``count`` is None, the count doesn't matter - the assertion is true
         if the text occurs at least once in the response.
         """
+
+        # If the response supports deferred rendering and hasn't been rendered
+        # yet, then ensure that it does get rendered before proceeding further.
+        if (hasattr(response, 'render') and callable(response.render)
+            and not response.is_rendered):
+            response.render()
+
         if msg_prefix:
             msg_prefix += ": "
 
@@ -523,6 +530,13 @@ class TransactionTestCase(SimpleTestCase):
         successfully, (i.e., the HTTP status code was as expected), and that
         ``text`` doesn't occurs in the content of the response.
         """
+
+        # If the response supports deferred rendering and hasn't been rendered
+        # yet, then ensure that it does get rendered before proceeding further.
+        if (hasattr(response, 'render') and callable(response.render)
+            and not response.is_rendered):
+            response.render()
+
         if msg_prefix:
             msg_prefix += ": "
 
diff --git a/tests/regressiontests/test_client_regress/models.py b/tests/regressiontests/test_client_regress/models.py
index ff851971ff7..b1e5bc4ff0d 100644
--- a/tests/regressiontests/test_client_regress/models.py
+++ b/tests/regressiontests/test_client_regress/models.py
@@ -14,6 +14,8 @@ import django.template.context
 from django.test import Client, TestCase
 from django.test.client import encode_file, RequestFactory
 from django.test.utils import ContextList, override_settings
+from django.template.response import SimpleTemplateResponse
+from django.http import HttpResponse
 
 
 class AssertContainsTests(TestCase):
@@ -130,6 +132,37 @@ class AssertContainsTests(TestCase):
         self.assertNotContains(r, u'はたけ')
         self.assertNotContains(r, '\xe3\x81\xaf\xe3\x81\x9f\xe3\x81\x91'.decode('utf-8'))
 
+    def test_assert_contains_renders_template_response(self):
+        """ Test that we can pass in an unrendered SimpleTemplateReponse
+            without throwing an error.
+            Refs #15826.
+        """
+        response = SimpleTemplateResponse(Template('Hello'), status=200)
+        self.assertContains(response, 'Hello')
+
+    def test_assert_contains_using_non_template_response(self):
+        """ Test that auto-rendering does not affect responses that aren't
+            instances (or subclasses) of SimpleTemplateResponse.
+            Refs #15826.
+        """
+        response = HttpResponse('Hello')
+        self.assertContains(response, 'Hello')
+
+    def test_assert_not_contains_renders_template_response(self):
+        """ Test that we can pass in an unrendered SimpleTemplateReponse
+            without throwing an error.
+            Refs #15826.
+        """
+        response = SimpleTemplateResponse(Template('Hello'), status=200)
+        self.assertNotContains(response, 'Bye')
+
+    def test_assert_not_contains_using_non_template_response(self):
+        """ Test that auto-rendering does not affect responses that aren't
+            instances (or subclasses) of SimpleTemplateResponse.
+            Refs #15826.
+        """
+        response = HttpResponse('Hello')
+        self.assertNotContains(response, 'Bye')
 
 class AssertTemplateUsedTests(TestCase):
     fixtures = ['testdata.json']