diff --git a/litellm/router.py b/litellm/router.py
index 24554e61516..fae51504610 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -1104,7 +1104,7 @@ class Router:
 
         self.validate_fallbacks(fallback_param=_fallbacks)
         ### set fallbacks
-        self.fallbacks = _fallbacks
+        self.fallbacks = list(_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/router_unit_tests/test_router_helper_utils.py b/tests/router_unit_tests/test_router_helper_utils.py
index d3ad1d989c8..8d847e812b3 100644
--- a/tests/router_unit_tests/test_router_helper_utils.py
+++ b/tests/router_unit_tests/test_router_helper_utils.py
@@ -1922,6 +1922,23 @@ 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(model_list):
+    fallbacks = [{"gpt-5-mini": ["gpt-5.5"]}]
+
+    first_router = Router(model_list=model_list, fallbacks=fallbacks, default_fallbacks=["gpt-5.5"])
+    second_router = Router(model_list=model_list, fallbacks=fallbacks, default_fallbacks=["gpt-image-1"])
+
+    assert fallbacks == [{"gpt-5-mini": ["gpt-5.5"]}]
+    assert first_router.fallbacks == [
+        {"gpt-5-mini": ["gpt-5.5"]},
+        {"*": ["gpt-5.5"]},
+    ]
+    assert second_router.fallbacks == [
+        {"gpt-5-mini": ["gpt-5.5"]},
+        {"*": ["gpt-image-1"]},
+    ]
+
+
 def test_add_optional_pre_call_checks(model_list):
     router = Router(model_list=model_list)