diff --git a/jupyter_client/jsonutil.py b/jupyter_client/jsonutil.py
index ad6ba6c8..be515fe0 100644
--- a/jupyter_client/jsonutil.py
+++ b/jupyter_client/jsonutil.py
@@ -9,7 +9,7 @@
 import warnings
 from binascii import b2a_base64
 from collections.abc import Iterable
-from datetime import date, datetime
+from datetime import date, datetime, timedelta
 from typing import Any, Union
 
 from dateutil.parser import isoparse as _dateutil_parse
@@ -126,6 +126,9 @@ def json_default(obj: Any) -> Any:
     if isinstance(obj, numbers.Real):
         return float(obj)
 
+    if isinstance(obj, timedelta):
+        return str(obj)
+
     raise TypeError("%r is not JSON serializable" % obj)
 
 
@@ -192,5 +195,8 @@ def json_clean(obj: Any) -> Any:
     if isinstance(obj, datetime | date):
         return obj.strftime(ISO8601)
 
+    if isinstance(obj, timedelta):
+        return str(obj)
+
     # we don't understand it, it's probably an unserializable object
     raise ValueError("Can't clean for JSON: %r" % obj)
diff --git a/tests/test_jsonutil.py b/tests/test_jsonutil.py
index 9f31dd6e..1f279b39 100644
--- a/tests/test_jsonutil.py
+++ b/tests/test_jsonutil.py
@@ -180,6 +180,7 @@ def test_json_default():
         (MyFloat(), 3.14),
         (MyInt(), 389),
         (datetime.date(2025, 4, 8), "2025-04-08"),
+        (datetime.timedelta(days=2, minutes=1, seconds=10), "2 days, 0:01:10"),
     ]
 
     for val, jval in pairs: