diff --git a/test_mount_subrouter.py b/test_mount_subrouter.py
new file mode 100644
index 0000000000..c5e845e9e3
--- /dev/null
+++ b/test_mount_subrouter.py
@@ -0,0 +1,80 @@
+"""Test for issue #10180: Mounting sub-applications under APIRouter."""
+from fastapi import FastAPI, APIRouter
+from starlette.testclient import TestClient
+
+
+def test_mount_subapp_under_router():
+    """Sub-application mounted under APIRouter should be reachable."""
+    app = FastAPI()
+    api_router = APIRouter(prefix="/api")
+
+    @api_router.get("/app")
+    def read_main():
+        return {"message": "Hello World from main app"}
+
+    app.include_router(api_router)
+
+    subapi = FastAPI()
+
+    @subapi.get("/sub")
+    def read_sub():
+        return {"message": "Hello World from sub API"}
+
+    api_router.mount("/subapi", subapi)
+
+    client = TestClient(app)
+
+    # Main app endpoint works
+    response = client.get("/api/app")
+    assert response.status_code == 200
+    assert response.json() == {"message": "Hello World from main app"}
+
+    # Sub-application endpoint should work (this is the bug)
+    response = client.get("/api/subapi/sub")
+    assert response.status_code == 200
+    assert response.json() == {"message": "Hello World from sub API"}
+
+
+def test_mount_subapp_docs():
+    """Sub-application should have its own OpenAPI docs."""
+    app = FastAPI()
+    api_router = APIRouter(prefix="/api")
+    app.include_router(api_router)
+
+    subapi = FastAPI()
+
+    @subapi.get("/sub")
+    def read_sub():
+        return {"message": "sub"}
+
+    api_router.mount("/subapi", subapi)
+
+    client = TestClient(app)
+
+    # Sub-app docs should be accessible
+    response = client.get("/api/subapi/docs")
+    assert response.status_code == 200
+
+
+def test_mount_subapp_after_include():
+    """Mounting a sub-app after include_router should still work."""
+    app = FastAPI()
+    api_router = APIRouter(prefix="/api")
+
+    @api_router.get("/main")
+    def read_main():
+        return {"ok": True}
+
+    # Mount sub-app BEFORE include_router
+    subapi = FastAPI()
+
+    @subapi.get("/endpoint")
+    def read_endpoint():
+        return {"sub": True}
+
+    api_router.mount("/sub", subapi)
+    app.include_router(api_router)
+
+    client = TestClient(app)
+    assert client.get("/api/main").status_code == 200
+    assert client.get("/api/sub/endpoint").status_code == 200