diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index b20d949689ec..cc417e2fbf9b 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -84,3 +84,6 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
 
         # MSC3786 (Add a default push rule to ignore m.room.server_acl events)
         self.msc3786_enabled: bool = experimental.get("msc3786_enabled", False)
+
+        # MSC3772: A push rule for mutual relations.
+        self.msc3772_enabled: bool = experimental.get("msc3772_enabled", False)
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index a59161ffd730..5122d7fe1061 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -120,6 +120,9 @@ def __init__(self, hs: "HomeServer"):
             resizable=False,
         )
 
+        # Whether to support MSC3772 is supported.
+        self._relations_match_enabled = self.hs.config.experimental.msc3772_enabled
+
     async def _get_rules_for_event(
         self, event: EventBase, context: EventContext
     ) -> Dict[str, List[Dict[str, Any]]]:
@@ -214,7 +217,12 @@ async def action_for_event_by_user(
         relations = await self.store.get_mutual_event_relations(event)
 
         evaluator = PushRuleEvaluatorForEvent(
-            event, len(room_members), sender_power_level, power_levels, relations
+            event,
+            len(room_members),
+            sender_power_level,
+            power_levels,
+            relations,
+            self._relations_match_enabled,
         )
 
         # If the event is not a state event check if any users ignore the sender.
diff --git a/synapse/push/push_rule_evaluator.py b/synapse/push/push_rule_evaluator.py
index 18516fae7ec5..182c4f6cf1f4 100644
--- a/synapse/push/push_rule_evaluator.py
+++ b/synapse/push/push_rule_evaluator.py
@@ -121,12 +121,14 @@ def __init__(
         sender_power_level: int,
         power_levels: Dict[str, Union[int, Dict[str, int]]],
         relations: Set[Tuple[str, str, str]],
+        relations_match_enabled: bool,
     ):
         self._event = event
         self._room_member_count = room_member_count
         self._sender_power_level = sender_power_level
         self._power_levels = power_levels
         self._relations = relations
+        self._relations_match_enabled = relations_match_enabled
 
         # Maps strings of e.g. 'content.body' -> event["content"]["body"]
         self._value_cache = _flatten_dict(event)
@@ -190,7 +192,10 @@ def matches(
             return _sender_notification_permission(
                 self._event, condition, self._sender_power_level, self._power_levels
             )
-        elif condition["kind"] == "org.matrix.msc3772.relation_match":
+        elif (
+            condition["kind"] == "org.matrix.msc3772.relation_match"
+            and self._relations_match_enabled
+        ):
             return self._relation_match(condition, user_id)
         else:
             return True
diff --git a/synapse/storage/databases/main/push_rule.py b/synapse/storage/databases/main/push_rule.py
index 4ed913e24879..708cc84da279 100644
--- a/synapse/storage/databases/main/push_rule.py
+++ b/synapse/storage/databases/main/push_rule.py
@@ -54,6 +54,11 @@ def _is_experimental_rule_enabled(
         and not experimental_config.msc3786_enabled
     ):
         return False
+    if (
+        rule_id == "global/underride/.org.matrix.msc3772.thread_reply"
+        and not experimental_config.msc3772_enabled
+    ):
+        return False
     return True
 
 
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index 8980836bd1a7..d25dc49a3b1a 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -27,7 +27,10 @@
 
 class PushRuleEvaluatorTestCase(unittest.TestCase):
     def _get_evaluator(
-        self, content: JsonDict, relations: Optional[Set[Tuple[str, str, str]]] = None
+        self,
+        content: JsonDict,
+        relations: Optional[Set[Tuple[str, str, str]]] = None,
+        relations_match_enabled: bool = False,
     ) -> PushRuleEvaluatorForEvent:
         event = FrozenEvent(
             {
@@ -49,6 +52,7 @@ def _get_evaluator(
             sender_power_level,
             power_levels,
             relations or set(),
+            relations_match_enabled,
         )
 
     def test_display_name(self) -> None:
@@ -285,9 +289,19 @@ def test_tweaks_for_actions(self) -> None:
 
     def test_relation_match(self) -> None:
         """Test the relation_match push rule kind."""
+
+        # Check if the experimental feature is disabled.
         evaluator = self._get_evaluator(
             {}, {("m.annotation", "@user:test", "m.reaction")}
         )
+        condition = {"kind": "relation_match"}
+        # Oddly, an unknown condition always matches.
+        self.assertTrue(evaluator.matches(condition, "@user:test", "foo"))
+
+        # A push rule evaluator with the experimental rule enabled.
+        evaluator = self._get_evaluator(
+            {}, {("m.annotation", "@user:test", "m.reaction")}, True
+        )
 
         # Check just relation type.
         condition = {