diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index 0432db2f0..77c87228a 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -1444,7 +1444,7 @@ def output_json(self, *, finished=False, **kwargs):
             finished=finished,
             time=time.time(),
         ))
-        print(json.dumps(kwargs), file=sys.stderr)
+        print(json.dumps(kwargs), file=sys.stderr, flush=True)
 
     def finish(self):
         if self.json: