diff --git a/django/conf/__init__.py b/django/conf/__init__.py
index f63df722c2..72ec964d23 100644
--- a/django/conf/__init__.py
+++ b/django/conf/__init__.py
@@ -271,8 +271,9 @@ class Settings:
                 raise ImproperlyConfigured(
                     "DEFAULT_FILE_STORAGE/STORAGES are mutually exclusive."
                 )
-            self.STORAGES[DEFAULT_STORAGE_ALIAS] = {
-                "BACKEND": self.DEFAULT_FILE_STORAGE
+            self.STORAGES = {
+                **self.STORAGES,
+                DEFAULT_STORAGE_ALIAS: {"BACKEND": self.DEFAULT_FILE_STORAGE},
             }
             warnings.warn(DEFAULT_FILE_STORAGE_DEPRECATED_MSG, RemovedInDjango51Warning)
 
@@ -281,8 +282,9 @@ class Settings:
                 raise ImproperlyConfigured(
                     "STATICFILES_STORAGE/STORAGES are mutually exclusive."
                 )
-            self.STORAGES[STATICFILES_STORAGE_ALIAS] = {
-                "BACKEND": self.STATICFILES_STORAGE
+            self.STORAGES = {
+                **self.STORAGES,
+                STATICFILES_STORAGE_ALIAS: {"BACKEND": self.STATICFILES_STORAGE},
             }
             warnings.warn(STATICFILES_STORAGE_DEPRECATED_MSG, RemovedInDjango51Warning)
         # RemovedInDjango51Warning.
diff --git a/docs/releases/4.2.6.txt b/docs/releases/4.2.6.txt
index 87bb28183e..23d5f2a04f 100644
--- a/docs/releases/4.2.6.txt
+++ b/docs/releases/4.2.6.txt
@@ -9,4 +9,6 @@ Django 4.2.6 fixes several bugs in 4.2.5.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 4.2.5 where overriding the deprecated
+  ``DEFAULT_FILE_STORAGE`` and ``STATICFILES_STORAGE`` settings in tests caused
+  the main ``STORAGES`` to mutate (:ticket:`34821`).
diff --git a/tests/deprecation/test_storages.py b/tests/deprecation/test_storages.py
index 0574f3e880..99a1fc1884 100644
--- a/tests/deprecation/test_storages.py
+++ b/tests/deprecation/test_storages.py
@@ -32,6 +32,7 @@ class StaticfilesStorageDeprecationTests(TestCase):
                 pass
 
     def test_settings_init(self):
+        old_staticfiles_storage = settings.STORAGES.get(STATICFILES_STORAGE_ALIAS)
         settings_module = ModuleType("fake_settings_module")
         settings_module.USE_TZ = True
         settings_module.STATICFILES_STORAGE = (
@@ -49,6 +50,11 @@ class StaticfilesStorageDeprecationTests(TestCase):
                     ),
                 },
             )
+            # settings.STORAGES is not mutated.
+            self.assertEqual(
+                settings.STORAGES.get(STATICFILES_STORAGE_ALIAS),
+                old_staticfiles_storage,
+            )
         finally:
             del sys.modules["fake_settings_module"]
 
@@ -161,6 +167,7 @@ class DefaultStorageDeprecationTests(TestCase):
                 pass
 
     def test_settings_init(self):
+        old_default_storage = settings.STORAGES.get(DEFAULT_STORAGE_ALIAS)
         settings_module = ModuleType("fake_settings_module")
         settings_module.USE_TZ = True
         settings_module.DEFAULT_FILE_STORAGE = "django.core.files.storage.Storage"
@@ -172,6 +179,11 @@ class DefaultStorageDeprecationTests(TestCase):
                 fake_settings.STORAGES[DEFAULT_STORAGE_ALIAS],
                 {"BACKEND": "django.core.files.storage.Storage"},
             )
+            # settings.STORAGES is not mutated.
+            self.assertEqual(
+                settings.STORAGES.get(DEFAULT_STORAGE_ALIAS),
+                old_default_storage,
+            )
         finally:
             del sys.modules["fake_settings_module"]