diff --git a/fastapi/_compat.py b/fastapi/_compat.py
index c07e4a3b0..fa1c5604b 100644
--- a/fastapi/_compat.py
+++ b/fastapi/_compat.py
@@ -28,7 +28,6 @@ from typing_extensions import Annotated, Literal, get_args, get_origin
 PYDANTIC_VERSION_MINOR_TUPLE = tuple(int(x) for x in PYDANTIC_VERSION.split(".")[:2])
 PYDANTIC_V2 = PYDANTIC_VERSION_MINOR_TUPLE[0] == 2
 
-
 sequence_annotation_to_type = {
     Sequence: list,
     List: list,
@@ -108,9 +107,14 @@ if PYDANTIC_V2:
             return self.field_info.annotation
 
         def __post_init__(self) -> None:
-            self._type_adapter: TypeAdapter[Any] = TypeAdapter(
-                Annotated[self.field_info.annotation, self.field_info]
-            )
+            from pydantic import PydanticDeprecatedSince20
+
+            try:
+                self._type_adapter: TypeAdapter[Any] = TypeAdapter(
+                    Annotated[self.field_info.annotation, self.field_info]
+                )
+            except PydanticDeprecatedSince20:
+                pass
 
         def get_default(self) -> Any:
             if self.field_info.is_required():
@@ -133,6 +137,16 @@ if PYDANTIC_V2:
                 return None, _regenerate_error_with_loc(
                     errors=exc.errors(include_url=False), loc_prefix=loc
                 )
+            except AttributeError:
+                # pydantic v1
+                from pydantic import v1
+
+                try:
+                    return v1.parse_obj_as(self.type_, value), None
+                except v1.ValidationError as exc:
+                    return None, _regenerate_error_with_loc(
+                        errors=exc.errors(), loc_prefix=loc
+                    )
 
         def serialize(
             self,
@@ -146,18 +160,42 @@ if PYDANTIC_V2:
             exclude_defaults: bool = False,
             exclude_none: bool = False,
         ) -> Any:
-            # What calls this code passes a value that already called
-            # self._type_adapter.validate_python(value)
-            return self._type_adapter.dump_python(
-                value,
-                mode=mode,
-                include=include,
-                exclude=exclude,
-                by_alias=by_alias,
-                exclude_unset=exclude_unset,
-                exclude_defaults=exclude_defaults,
-                exclude_none=exclude_none,
-            )
+            try:
+                # What calls this code passes a value that already called
+                # self._type_adapter.validate_python(value)
+                return self._type_adapter.dump_python(
+                    value,
+                    mode=mode,
+                    include=include,
+                    exclude=exclude,
+                    by_alias=by_alias,
+                    exclude_unset=exclude_unset,
+                    exclude_defaults=exclude_defaults,
+                    exclude_none=exclude_none,
+                )
+            except AttributeError:
+                # pydantic v1
+                try:
+                    return value.dict(
+                        include=include,
+                        exclude=exclude,
+                        by_alias=by_alias,
+                        exclude_unset=exclude_unset,
+                        exclude_defaults=exclude_defaults,
+                        exclude_none=exclude_none,
+                    )
+                except AttributeError:
+                    return [
+                        item.dict(
+                            include=include,
+                            exclude=exclude,
+                            by_alias=by_alias,
+                            exclude_unset=exclude_unset,
+                            exclude_defaults=exclude_defaults,
+                            exclude_none=exclude_none,
+                        )
+                        for item in value
+                    ]
 
         def __hash__(self) -> int:
             # Each ModelField is unique for our purposes, to allow making a dict from
diff --git a/tests/test_pydantic_v1_models.py b/tests/test_pydantic_v1_models.py
new file mode 100644
index 000000000..2728d9f15
--- /dev/null
+++ b/tests/test_pydantic_v1_models.py
@@ -0,0 +1,99 @@
+from typing import List, Optional
+
+import pytest
+from fastapi import Body, FastAPI
+from fastapi._compat import PYDANTIC_V2
+from fastapi.exceptions import ResponseValidationError
+from fastapi.testclient import TestClient
+from typing_extensions import Annotated
+
+from tests.utils import needs_pydanticv2
+
+if PYDANTIC_V2:
+    from pydantic import v1
+
+    class Item(v1.BaseModel):
+        name: str
+        description: Optional[str] = None
+        price: float
+        tax: Optional[float] = None
+        tags: list = []
+
+    class Model(v1.BaseModel):
+        name: str
+
+else:
+    from pydantic import BaseModel
+
+    class Item(BaseModel):
+        name: str
+        description: Optional[str] = None
+        price: float
+        tax: Optional[float] = None
+        tags: list = []
+
+    class Model(BaseModel):
+        name: str
+
+
+app = FastAPI()
+
+
+@app.post("/request_body")
+async def request_body(body: Annotated[Item, Body()]):
+    return body
+
+
+@app.get("/response_model", response_model=Model)
+async def response_model():
+    return Model(name="valid_model")
+
+
+@app.get("/response_model__invalid", response_model=Model)
+async def response_model__invalid():
+    return 1
+
+
+@app.get("/response_model_list", response_model=List[Model])
+async def response_model_list():
+    return [Model(name="valid_model")]
+
+
+@app.get("/response_model_list__invalid", response_model=List[Model])
+async def response_model_list__invalid():
+    return [1]
+
+
+client = TestClient(app)
+
+
+@needs_pydanticv2
+class TestResponseModel:
+    def test_simple__valid(self):
+        response = client.get("/response_model")
+        assert response.status_code == 200
+        assert response.json() == {"name": "valid_model"}
+
+    def test_simple__invalid(self):
+        with pytest.raises(ResponseValidationError):
+            client.get("/response_model__invalid")
+
+    def test_list__valid(self):
+        response = client.get("/response_model_list")
+        assert response.status_code == 200
+        assert response.json() == [{"name": "valid_model"}]
+
+    def test_list__invalid(self):
+        with pytest.raises(ResponseValidationError):
+            client.get("/response_model_list__invalid")
+
+
+@needs_pydanticv2
+class TestRequestBody:
+    def test_model__valid(self):
+        response = client.post("/request_body", json={"name": "myname", "price": 1.0})
+        assert response.status_code == 200, response.text
+
+    def test_model__invalid(self):
+        response = client.post("/request_body", json={"name": "myname"})
+        assert response.status_code == 422, response.text