diff --git a/fastapi/_compat/shared.py b/fastapi/_compat/shared.py
index 56e4ce46c..4b55ca25b 100644
--- a/fastapi/_compat/shared.py
+++ b/fastapi/_compat/shared.py
@@ -183,3 +183,18 @@ def is_uploadfile_sequence_annotation(annotation: Any) -> bool:
         is_uploadfile_or_nonable_uploadfile_annotation(sub_annotation)
         for sub_annotation in get_args(annotation)
     )
+
+
+def annotation_is_pydantic_v1(annotation: Any) -> bool:
+    if lenient_issubclass(annotation, v1.BaseModel):
+        return True
+    origin = get_origin(annotation)
+    if origin is Union or origin is UnionType:
+        for arg in get_args(annotation):
+            if lenient_issubclass(arg, v1.BaseModel):
+                return True
+    if field_annotation_is_sequence(annotation):
+        for sub_annotation in get_args(annotation):
+            if annotation_is_pydantic_v1(sub_annotation):
+                return True
+    return False