diff --git a/litellm/router.py b/litellm/router.py
index 99adc6c5fa6..198de5a70c8 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -1448,7 +1448,7 @@ class Router:
 
         self.validate_fallbacks(fallback_param=_fallbacks)
         ### set fallbacks
-        self.fallbacks = _fallbacks
+        self.fallbacks = _fallbacks[:] if _fallbacks is not None else None
 
         if default_fallbacks is not None or litellm.default_fallbacks is not None:
             _fallbacks = default_fallbacks or litellm.default_fallbacks
diff --git a/tests/unit/test_router/test_router_helper_utils.py b/tests/unit/test_router/test_router_helper_utils.py
index 4ac917d706d..876b1ab5b87 100644
--- a/tests/unit/test_router/test_router_helper_utils.py
+++ b/tests/unit/test_router/test_router_helper_utils.py
@@ -1646,6 +1646,26 @@ def test_has_default_fallbacks(model_list, has_default_fallbacks, expected_resul
     assert router._has_default_fallbacks() is expected_result
 
 
+def test_default_fallbacks_do_not_mutate_input_fallbacks() -> None:
+    fallbacks: Final[list[dict[str, list[str]]]] = [{"primary": ["secondary"]}]
+    first_router: Final = Router(model_list=[], fallbacks=fallbacks, default_fallbacks=["first-default"])
+    second_router: Final = Router(model_list=[], fallbacks=fallbacks, default_fallbacks=["second-default"])
+
+    try:
+        assert fallbacks == [{"primary": ["secondary"]}]
+        assert first_router.fallbacks == [
+            {"primary": ["secondary"]},
+            {"*": ["first-default"]},
+        ]
+        assert second_router.fallbacks == [
+            {"primary": ["secondary"]},
+            {"*": ["second-default"]},
+        ]
+    finally:
+        first_router.discard()
+        second_router.discard()
+
+
 def test_add_optional_pre_call_checks(model_list):
     router = Router(model_list=model_list)