diff --git a/src/assertical/fixtures/fastapi.py b/src/assertical/fixtures/fastapi.py
index da511ee..91436b1 100644
--- a/src/assertical/fixtures/fastapi.py
+++ b/src/assertical/fixtures/fastapi.py
@@ -5,7 +5,7 @@
 import uvicorn
 from asgi_lifespan import LifespanManager
 from fastapi import FastAPI
-from httpx import ASGITransport, AsyncClient
+from httpx import ASGITransport, AsyncClient, Request, URL
 from httpx._types import AuthTypes
 
 
@@ -41,12 +41,35 @@ async def down(self) -> None:
             await self._serve_task
 
 
+class ContentTypeAsyncClient(AsyncClient):
+    """AsyncClient implementation that assigns content-type for bodied requests."""
+
+    def __init__(self, *, content_type: str, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.content_type = content_type
+
+    def build_request(self, method: str, url: URL | str, **kwargs: Any) -> Request:
+        """Overloaded implementation from parent class.
+
+        This detects if there is a body associated with the request and sets a content-type
+        """
+        request = super().build_request(method, url, **kwargs)
+
+        if request.content:
+            if "content-type" not in request.headers:
+                request.headers["Content-Type"] = self.content_type
+
+        return request
+
+
 @asynccontextmanager
 async def start_app_with_client(
     app: FastAPI, client_auth: Optional[AuthTypes] = None
 ) -> AsyncGenerator[AsyncClient, None]:
     """Creates an AsyncClient for a test app and returns it as an AsyncGenerator for use with a fixture:
 
+    The default content-type for bodied requests is application/json
+
     Usage:
 
     async with start_app_with_client(app) as client:
@@ -59,7 +82,9 @@ async def start_app_with_client(
     """
 
     async with LifespanManager(app):  # This ensures that startup events are fired when the app starts
-        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test", auth=client_auth) as c:
+        async with ContentTypeAsyncClient(
+            content_type="application/json", transport=ASGITransport(app=app), base_url="http://test", auth=client_auth
+        ) as c:
             yield c
 
 
diff --git a/tests/fixtures/test_fastapi.py b/tests/fixtures/test_fastapi.py
index 3091a01..3c4f8bb 100644
--- a/tests/fixtures/test_fastapi.py
+++ b/tests/fixtures/test_fastapi.py
@@ -1,4 +1,5 @@
 from contextlib import asynccontextmanager
+import json
 
 import pytest
 from fastapi import FastAPI
@@ -29,8 +30,25 @@ async def added_later():
     async def hello_world():
         return {"msg": "Hello World"}
 
+    @test_app.post("/hello_world")
+    async def hello_world_create():
+        return None
+
     return test_app
 
+@pytest.mark.anyio
+async def test_client_content_type_set():
+    """Ensures that a request content-type header is only set for bodied requests."""
+    app = generate_test_app()
+
+    async with start_app_with_client(app) as client:
+        body = json.dumps({"msg": "hello world"})
+        response = await client.post("/hello_world", content=body)
+        assert response.request.headers.get("content-type") == "application/json"
+        
+        response = await client.get("/hello_world")
+        assert not response.request.headers.get("content-type")
+
 
 @pytest.mark.parametrize("multi_iteration", [1, 2, 3])
 @pytest.mark.anyio