diff --git a/modules/restricted-guests/synapse/tox.ini b/modules/restricted-guests/synapse/tox.ini
index 0da1d491aa..33f6df7e3c 100644
--- a/modules/restricted-guests/synapse/tox.ini
+++ b/modules/restricted-guests/synapse/tox.ini
@@ -6,6 +6,11 @@ isolated_build = true
 
 [testenv:py]
 
+deps =
+  # Temp: downgrade prometheus-client until Synapse is compatible with 0.24.0+
+  # See https://github.com/element-hq/synapse/issues/19375
+  prometheus-client==0.23.1
+
 extras = dev
 
 commands =