diff --git a/django/template/loader.py b/django/template/loader.py
index 279d8e86fe..e58dc3231d 100644
--- a/django/template/loader.py
+++ b/django/template/loader.py
@@ -179,11 +179,15 @@ def render_to_string(template_name, dictionary=None, context_instance=None):
         t = select_template(template_name)
     else:
         t = get_template(template_name)
-    if context_instance:
-        context_instance.update(dictionary)
-    else:
-        context_instance = Context(dictionary)
-    return t.render(context_instance)
+    if not context_instance:
+        return t.render(Context(dictionary))
+    # Add the dictionary to the context stack, ensuring it gets removed again
+    # to keep the context_instance in the same state it started in.
+    context_instance.update(dictionary)
+    try:
+        return t.render(context_instance)
+    finally:
+        context_instance.pop()
 
 def select_template(template_name_list):
     "Given a list of template names, returns the first that can be loaded."
diff --git a/tests/regressiontests/templates/loaders.py b/tests/regressiontests/templates/loaders.py
index f0759ec2c6..6c74033944 100644
--- a/tests/regressiontests/templates/loaders.py
+++ b/tests/regressiontests/templates/loaders.py
@@ -148,5 +148,30 @@ class CachedLoader(unittest.TestCase):
         # The two templates should not have the same content
         self.assertNotEqual(t1.render(Context({})), t2.render(Context({})))
 
+class RenderToStringTest(unittest.TestCase):
+
+    def setUp(self):
+        self._old_TEMPLATE_DIRS = settings.TEMPLATE_DIRS
+        settings.TEMPLATE_DIRS = (
+            os.path.join(os.path.dirname(__file__), 'templates'),
+        )
+
+    def tearDown(self):
+        settings.TEMPLATE_DIRS = self._old_TEMPLATE_DIRS
+
+    def test_basic(self):
+        self.assertEqual(loader.render_to_string('test_context.html'), 'obj:')
+
+    def test_basic_context(self):
+        self.assertEqual(loader.render_to_string('test_context.html',
+                                                 {'obj': 'test'}), 'obj:test')
+
+    def test_existing_context_kept_clean(self):
+        context = Context({'obj': 'before'})
+        output = loader.render_to_string('test_context.html', {'obj': 'after'},
+                                         context_instance=context)
+        self.assertEqual(output, 'obj:after')
+        self.assertEqual(context['obj'], 'before')
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/tests/regressiontests/templates/templates/test_context.html b/tests/regressiontests/templates/templates/test_context.html
new file mode 100644
index 0000000000..a100f03de6
--- /dev/null
+++ b/tests/regressiontests/templates/templates/test_context.html
@@ -0,0 +1 @@
+obj:{{ obj }}
\ No newline at end of file