diff --git a/jupyter_client/jsonutil.py b/jupyter_client/jsonutil.py
index 8532cc35..db46d1b1 100644
--- a/jupyter_client/jsonutil.py
+++ b/jupyter_client/jsonutil.py
@@ -111,7 +111,7 @@ def json_default(obj):
         return obj.isoformat().replace('+00:00', 'Z')
 
     if isinstance(obj, bytes):
-        return b2a_base64(obj).decode('ascii')
+        return b2a_base64(obj, newline=False).decode('ascii')
 
     if isinstance(obj, Iterable):
         return list(obj)
@@ -157,7 +157,7 @@ def json_clean(obj):
     if isinstance(obj, bytes):
         # unanmbiguous binary data is base64-encoded
         # (this probably should have happened upstream)
-        return b2a_base64(obj).decode('ascii')
+        return b2a_base64(obj, newline=False).decode('ascii')
 
     if isinstance(obj, container_to_list) or (
         hasattr(obj, '__iter__') and hasattr(obj, next_attr_name)
diff --git a/tests/test_jsonutil.py b/tests/test_jsonutil.py
index a3074b5c..3b029fcd 100644
--- a/tests/test_jsonutil.py
+++ b/tests/test_jsonutil.py
@@ -166,7 +166,7 @@ def test_json_default():
         (True, None),
         (False, None),
         (None, None),
-        ({"key": b"\xFF"}, {"key": "/w==\n"}),
+        ({"key": b"\xFF"}, {"key": "/w=="}),
         # Containers
         ([1, 2], None),
         ((1, 2), [1, 2]),