diff --git a/altair/__init__.py b/altair/__init__.py
index c48627520..8cf283e49 100644
--- a/altair/__init__.py
+++ b/altair/__init__.py
@@ -1,5 +1,5 @@
 # ruff: noqa
-__version__ = "5.4.0"
+__version__ = "5.5.0dev"
 
 # The content of __all__ is automatically written by
 # tools/update_init_file.py. Do not modify directly.
diff --git a/doc/conf.py b/doc/conf.py
index d024e8147..3e210a3fa 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -78,7 +78,7 @@
 # built documents.
 #
 # The short X.Y version.
-version = "5.4.0"
+version = "5.5.0dev"
 # The full version, including alpha/beta/rc tags.
 release = f"{version}"