diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index 61ca378dc1..8df7893714 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -76,6 +76,7 @@ REQUIREMENTS = [
     "netaddr>=0.7.18",
     "Jinja2>=2.9",
     "bleach>=1.4.2",
+    "html5lib>=1.0.0",
 ]
 
 CONDITIONAL_REQUIREMENTS = {