diff --git a/pymongo/_version.py b/pymongo/_version.py
index 6995a45526..8cca2a6eaa 100644
--- a/pymongo/_version.py
+++ b/pymongo/_version.py
@@ -18,7 +18,7 @@
 
 import re
 
-__version__ = "4.20.0"
+__version__ = "4.21.0.dev0"
 
 
 def get_version_tuple(version: str) -> tuple[int | str, ...]: