diff --git a/fastapi/routing.py b/fastapi/routing.py
index 36acb6b89d..b61bf3aa36 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -1819,6 +1819,12 @@ class APIRouter(routing.Router):
                 self.add_websocket_route(
                     prefix + route.path, route.endpoint, name=route.name
                 )
+            elif isinstance(route, routing.Mount):
+                self.mount(
+                    prefix + router.prefix + route.path,
+                    app=route.app,
+                    name=route.name,
+                )
         for handler in router.on_startup:
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown: