diff --git a/fastapi/routing.py b/fastapi/routing.py
index 24fc77f81..1ca32a237 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -29,6 +29,7 @@ from fastapi._compat import (
     _model_dump,
     _normalize_errors,
     lenient_issubclass,
+    may_v1,
 )
 from fastapi.datastructures import Default, DefaultPlaceholder
 from fastapi.dependencies.models import Dependant
@@ -152,7 +153,7 @@ def _prepare_response_content(
     exclude_defaults: bool = False,
     exclude_none: bool = False,
 ) -> Any:
-    if isinstance(res, BaseModel):
+    if isinstance(res, (may_v1.BaseModel, BaseModel)):
         read_with_orm_mode = getattr(_get_model_config(res), "read_with_orm_mode", None)
         if read_with_orm_mode:
             # Let from_orm extract the data from this model instead of converting