diff --git a/IPython/nbformat/v4/nbjson.py b/IPython/nbformat/v4/nbjson.py
index 753d49a..5430512 100644
--- a/IPython/nbformat/v4/nbjson.py
+++ b/IPython/nbformat/v4/nbjson.py
@@ -49,6 +49,7 @@ class JSONWriter(NotebookWriter):
         kwargs['indent'] = 1
         kwargs['sort_keys'] = True
         kwargs['separators'] = (',',': ')
+        kwargs.setdefault('ensure_ascii', False)
         # don't modify in-memory dict
         nb = copy.deepcopy(nb)
         if kwargs.pop('split_lines', True):