diff --git a/CHANGES.md b/CHANGES.md
index 206859cff..a41abbefb 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,12 @@
+Synapse 1.33.1 (2021-05-06)
+===========================
+
+Bugfixes
+--------
+
+- Fix bug where `/sync` would break if using the latest version of `attrs` dependency, by pinning to a previous version. ([\#9937](https://github.com/matrix-org/synapse/issues/9937))
+
+
 Synapse 1.33.0 (2021-05-05)
 ===========================
 
diff --git a/debian/changelog b/debian/changelog
index b54d3f2bc..de50dd14e 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,9 @@
+matrix-synapse-py3 (1.33.1) stable; urgency=medium
+
+  * New synapse release 1.33.1.
+
+ -- Synapse Packaging team <packages@matrix.org>  Thu, 06 May 2021 14:06:33 +0100
+
 matrix-synapse-py3 (1.33.0) stable; urgency=medium
 
   * New synapse release 1.33.0.
diff --git a/synapse/__init__.py b/synapse/__init__.py
index 5eac40730..441cd8b33 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -47,7 +47,7 @@ try:
 except ImportError:
     pass
 
-__version__ = "1.33.0"
+__version__ = "1.33.1"
 
 if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     # We import here so that we don't have to install a bunch of deps when
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index d58eeeaa7..2113c620c 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -78,7 +78,8 @@ REQUIREMENTS = [
     # we use attr.validators.deep_iterable, which arrived in 19.1.0 (Note:
     # Fedora 31 only has 19.1, so if we want to upgrade we should wait until 33
     # is out in November.)
-    "attrs>=19.1.0",
+    # Note: 21.1.0 broke `/sync`, see #9936
+    "attrs>=19.1.0,<21.1.0",
     "netaddr>=0.7.18",
     "Jinja2>=2.9",
     "bleach>=1.4.3",