diff --git a/changelog.d/8527.bugfix b/changelog.d/8527.bugfix
new file mode 100644
index 000000000000..727e0ba2992a
--- /dev/null
+++ b/changelog.d/8527.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in v1.7.0 that could cause Synapse to insert values from non-state `m.room.retention` events into the `room_retention` database table.
diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index 9df35b54baff..5f9af8529be0 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -83,6 +83,9 @@ def _validate_retention(self, event):
         Args:
             event (FrozenEvent): The event to validate.
         """
+        if not event.is_state():
+            raise SynapseError(code=400, msg="must be a state event")
+
         min_lifetime = event.content.get("min_lifetime")
         max_lifetime = event.content.get("max_lifetime")
 
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 18def01f5041..f766e148a05e 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1241,6 +1241,10 @@ def _store_room_message_txn(self, txn, event):
             )
 
     def _store_retention_policy_for_room_txn(self, txn, event):
+        if not event.is_state():
+            logger.debug("Ignoring non-state m.room.retention event")
+            return
+
         if hasattr(event, "content") and (
             "min_lifetime" in event.content or "max_lifetime" in event.content
         ):