diff --git a/CHANGELOG.md b/CHANGELOG.md
index c063c0814a..2d8634f044 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
 
+### Fixed
+
+* Fix `app` type signature in `ASGITransport`. (#3109)
+
 ## 0.27.0 (21st February, 2024)
 
 ### Deprecated
diff --git a/httpx/_transports/asgi.py b/httpx/_transports/asgi.py
index 794cb17bc8..d1828f254d 100644
--- a/httpx/_transports/asgi.py
+++ b/httpx/_transports/asgi.py
@@ -16,13 +16,13 @@
     Event = typing.Union[asyncio.Event, trio.Event]
 
 
-_Message = typing.Dict[str, typing.Any]
+_Message = typing.MutableMapping[str, typing.Any]
 _Receive = typing.Callable[[], typing.Awaitable[_Message]]
 _Send = typing.Callable[
-    [typing.Dict[str, typing.Any]], typing.Coroutine[None, None, None]
+    [typing.MutableMapping[str, typing.Any]], typing.Awaitable[None]
 ]
 _ASGIApp = typing.Callable[
-    [typing.Dict[str, typing.Any], _Receive, _Send], typing.Coroutine[None, None, None]
+    [typing.MutableMapping[str, typing.Any], _Receive, _Send], typing.Awaitable[None]
 ]
 
 __all__ = ["ASGITransport"]
@@ -141,7 +141,7 @@ async def receive() -> dict[str, typing.Any]:
                 return {"type": "http.request", "body": b"", "more_body": False}
             return {"type": "http.request", "body": body, "more_body": True}
 
-        async def send(message: dict[str, typing.Any]) -> None:
+        async def send(message: typing.MutableMapping[str, typing.Any]) -> None:
             nonlocal status_code, response_headers, response_started
 
             if message["type"] == "http.response.start":