diff --git a/django/contrib/admin/options.py b/django/contrib/admin/options.py
index 12487dcc9b..75c256963b 100644
--- a/django/contrib/admin/options.py
+++ b/django/contrib/admin/options.py
@@ -1785,7 +1785,7 @@ class ModelAdmin(BaseModelAdmin):
             }
             if request.method == 'POST':
                 formset_params.update({
-                    'data': request.POST,
+                    'data': request.POST.copy(),
                     'files': request.FILES,
                     'save_as_new': '_saveasnew' in request.POST
                 })
diff --git a/django/contrib/auth/admin.py b/django/contrib/auth/admin.py
index f84652f8b4..f3c69023be 100644
--- a/django/contrib/auth/admin.py
+++ b/django/contrib/auth/admin.py
@@ -205,6 +205,7 @@ class UserAdmin(admin.ModelAdmin):
         # * The user has pressed the 'Save and add another' button
         # * We are adding a user in a popup
         if '_addanother' not in request.POST and IS_POPUP_VAR not in request.POST:
+            request.POST = request.POST.copy()
             request.POST['_continue'] = 1
         return super(UserAdmin, self).response_add(request, obj,
                                                    post_url_continue)
diff --git a/django/http/multipartparser.py b/django/http/multipartparser.py
index 87d80bdde5..616185f021 100644
--- a/django/http/multipartparser.py
+++ b/django/http/multipartparser.py
@@ -289,6 +289,7 @@ class MultiPartParser(object):
             if retval:
                 break
 
+        self._post._mutable = False
         return self._post, self._files
 
     def handle_file_complete(self, old_field_name, counters):
diff --git a/docs/releases/1.11.txt b/docs/releases/1.11.txt
index 7bf206c678..59b7fb54f9 100644
--- a/docs/releases/1.11.txt
+++ b/docs/releases/1.11.txt
@@ -634,6 +634,10 @@ Miscellaneous
 
 * The undocumented ``django.utils.functional.lazy_property`` is removed.
 
+* For consistency with non-multipart requests, ``MultiPartParser.parse()`` now
+  leaves ``request.POST`` immutable. If you're modifying that ``QueryDict``,
+  you must now first copy it, e.g. ``request.POST.copy()``.
+
 .. _deprecated-features-1.11:
 
 Features deprecated in 1.11
diff --git a/tests/requests/tests.py b/tests/requests/tests.py
index 0942619715..27dc439e5f 100644
--- a/tests/requests/tests.py
+++ b/tests/requests/tests.py
@@ -533,6 +533,25 @@ class RequestsTests(SimpleTestCase):
         self.assertEqual(request.read(13), b'--boundary\r\nC')
         self.assertEqual(request.POST, {'name': ['value']})
 
+    def test_POST_immutable_for_mutipart(self):
+        """
+        MultiPartParser.parse() leaves request.POST immutable.
+        """
+        payload = FakePayload("\r\n".join([
+            '--boundary',
+            'Content-Disposition: form-data; name="name"',
+            '',
+            'value',
+            '--boundary--',
+        ]))
+        request = WSGIRequest({
+            'REQUEST_METHOD': 'POST',
+            'CONTENT_TYPE': 'multipart/form-data; boundary=boundary',
+            'CONTENT_LENGTH': len(payload),
+            'wsgi.input': payload,
+        })
+        self.assertFalse(request.POST._mutable)
+
     def test_POST_connection_error(self):
         """
         If wsgi.input.read() raises an exception while trying to read() the