diff --git a/modules/restricted-guests/synapse/scripts/run_in_venv.sh b/modules/restricted-guests/synapse/scripts/run_in_venv.sh
index 23adf1e03b..fee32d2fa1 100755
--- a/modules/restricted-guests/synapse/scripts/run_in_venv.sh
+++ b/modules/restricted-guests/synapse/scripts/run_in_venv.sh
@@ -20,6 +20,10 @@ venv_path=${repo_root:-./}.venv
 if [ ! -d "$venv_path" ]; then
     "python${PYTHON_VERSION:-3}" -m venv "$venv_path"
     "$venv_path/bin/pip" install -e '.[dev]'
+
+    # Temp: downgrade prometheus-client until Synapse is compatible with 0.24.0+
+    # See https://github.com/element-hq/synapse/issues/19375
+    "$venv_path/bin/pip" install --force prometheus-client==0.23.1
 fi
 
 [ $# -gt 1 ] && . "$venv_path/bin/activate" && "$@"
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 =