diff --git a/django/conf/__init__.py b/django/conf/__init__.py
index 608ad256dc..3337fd6f13 100644
--- a/django/conf/__init__.py
+++ b/django/conf/__init__.py
@@ -104,6 +104,8 @@ class LazySettings(LazyObject):
             raise RuntimeError('Settings already configured.')
         holder = UserSettingsHolder(default_settings)
         for name, value in options.items():
+            if not name.isupper():
+                raise TypeError('Setting %r must be uppercase.' % name)
             setattr(holder, name, value)
         self._wrapped = holder
 
@@ -198,7 +200,7 @@ class UserSettingsHolder:
         self.default_settings = default_settings
 
     def __getattr__(self, name):
-        if name in self._deleted:
+        if not name.isupper() or name in self._deleted:
             raise AttributeError
         return getattr(self.default_settings, name)
 
diff --git a/tests/settings_tests/tests.py b/tests/settings_tests/tests.py
index bfd580b600..280f46f2aa 100644
--- a/tests/settings_tests/tests.py
+++ b/tests/settings_tests/tests.py
@@ -1,7 +1,7 @@
 import os
 import sys
 import unittest
-from types import ModuleType
+from types import ModuleType, SimpleNamespace
 from unittest import mock
 
 from django.conf import ENVIRONMENT_VARIABLE, LazySettings, Settings, settings
@@ -318,6 +318,17 @@ class SettingsTests(SimpleTestCase):
         with self.assertRaisesMessage(RuntimeError, 'Settings already configured.'):
             settings.configure()
 
+    def test_nonupper_settings_prohibited_in_configure(self):
+        s = LazySettings()
+        with self.assertRaisesMessage(TypeError, "Setting 'foo' must be uppercase."):
+            s.configure(foo='bar')
+
+    def test_nonupper_settings_ignored_in_default_settings(self):
+        s = LazySettings()
+        s.configure(SimpleNamespace(foo='bar'))
+        with self.assertRaises(AttributeError):
+            getattr(s, 'foo')
+
     @requires_tz_support
     @mock.patch('django.conf.global_settings.TIME_ZONE', 'test')
     def test_incorrect_timezone(self):