diff --git a/changelog.d/7978.bugfix b/changelog.d/7978.bugfix
new file mode 100644
index 000000000000..247b18db20ba
--- /dev/null
+++ b/changelog.d/7978.bugfix
@@ -0,0 +1 @@
+Fix a long standing bug: 'Duplicate key value violates unique constraint "event_relations_id"' when message retention is configured.
diff --git a/synapse/storage/data_stores/main/purge_events.py b/synapse/storage/data_stores/main/purge_events.py
index 65465691391d..b53fe35c338b 100644
--- a/synapse/storage/data_stores/main/purge_events.py
+++ b/synapse/storage/data_stores/main/purge_events.py
@@ -62,6 +62,7 @@ def _purge_history_txn(self, txn, room_id, token_str, delete_local_events):
         #     event_json
         #     event_push_actions
         #     event_reference_hashes
+        #     event_relations
         #     event_search
         #     event_to_state_groups
         #     events
@@ -209,6 +210,7 @@ def _purge_history_txn(self, txn, room_id, token_str, delete_local_events):
             "event_edges",
             "event_forward_extremities",
             "event_reference_hashes",
+            "event_relations",
             "event_search",
             "rejections",
         ):