diff --git a/src/fastmcp/utilities/openapi/schemas.py b/src/fastmcp/utilities/openapi/schemas.py
index bb057f2eb..fa93b6c8d 100644
--- a/src/fastmcp/utilities/openapi/schemas.py
+++ b/src/fastmcp/utilities/openapi/schemas.py
@@ -123,6 +123,19 @@ def _replace_ref_with_defs(
             schema["additionalProperties"] = _replace_ref_with_defs(
                 additionalProperties
             )
+    # Handle propertyNames
+    if property_names := schema.get("propertyNames"):
+        if isinstance(property_names, dict):
+            schema["propertyNames"] = _replace_ref_with_defs(property_names)
+    # Handle patternProperties
+    if pattern_properties := schema.get("patternProperties"):
+        if isinstance(pattern_properties, dict):
+            schema["patternProperties"] = {
+                pattern: _replace_ref_with_defs(subschema)
+                if isinstance(subschema, dict)
+                else subschema
+                for pattern, subschema in pattern_properties.items()
+            }
     if info.get("description", description) and not schema.get("description"):
         schema["description"] = description
     return schema
diff --git a/tests/utilities/openapi/test_propertynames_ref_rewrite.py b/tests/utilities/openapi/test_propertynames_ref_rewrite.py
new file mode 100644
index 000000000..809363410
--- /dev/null
+++ b/tests/utilities/openapi/test_propertynames_ref_rewrite.py
@@ -0,0 +1,34 @@
+from fastmcp.utilities.openapi.schemas import _replace_ref_with_defs
+
+
+def test_replace_ref_with_defs_rewrites_propertyNames_ref():
+    """
+    Regression test for issue #3303.
+
+    When using dict[StrEnum, Model], Pydantic generates:
+
+        {
+            "type": "object",
+            "propertyNames": {"$ref": "#/components/schemas/Category"},
+            "additionalProperties": {"$ref": "#/components/schemas/ItemInfo"}
+        }
+
+    _replace_ref_with_defs should rewrite BOTH refs to #/$defs/.
+    """
+
+    schema = {
+        "type": "object",
+        "propertyNames": {"$ref": "#/components/schemas/Category"},
+        "additionalProperties": {"$ref": "#/components/schemas/ItemInfo"},
+    }
+
+    result = _replace_ref_with_defs(schema)
+
+    # additionalProperties ref is rewritten
+    assert result["additionalProperties"]["$ref"] == "#/$defs/ItemInfo"
+
+    # propertyNames ref must also be rewritten (this was the bug)
+    assert result["propertyNames"]["$ref"] == "#/$defs/Category"
+
+    # Ensure no dangling OpenAPI refs remain
+    assert "#/components/schemas/" not in str(result)