diff --git a/fastapi/dependencies/utils.py b/fastapi/dependencies/utils.py
index 6ed3fcc55..7e50cb7c6 100644
--- a/fastapi/dependencies/utils.py
+++ b/fastapi/dependencies/utils.py
@@ -931,7 +931,7 @@ async def request_body_to_args(
 
     if (
         single_not_embedded_field
-        and resolves_to_basemodel(first_field.type_)
+        and lenient_issubclass(first_field.type_, BaseModel)
         and isinstance(received_body, FormData)
     ):
         fields_to_extract = get_cached_model_fields(first_field.type_)
diff --git a/tests/test_root_model.py b/tests/test_root_model.py
index 95a18f460..85a8e65d7 100644
--- a/tests/test_root_model.py
+++ b/tests/test_root_model.py
@@ -280,6 +280,10 @@ def test_root_model_union(left: Any, right: Any):
     def handler2(b: Annotated[Model, Body()]):
         return {"b": b}
 
+    @app2.post("/body2")
+    def handler2(q: Model) -> Model:
+        return q
+
     client2 = TestClient(app2)
     for val in ["2025-01-02", 42]:
         response = client2.get(f"/query?q={val}")
@@ -289,6 +293,15 @@ def test_root_model_union(left: Any, right: Any):
         assert response.status_code == 200, response.text
         assert response.json() == {"b": val}
 
+        # By default, RootModel is regarded as a body parameter
+        response = (
+            client2.post("/body2", json=val)
+            if issubclass(left, RootModel) or issubclass(right, RootModel)
+            else client2.post(f"/body2?q={val}")
+        )
+        assert response.status_code == 200, response.text
+        assert response.json() == val
+
     response = client2.get("/query?q=bad")
     assert response.status_code == 422, response.text
     assert {detail["msg"] for detail in response.json()["detail"]} == {