diff --git a/changelog.d/15138.misc b/changelog.d/15138.misc
new file mode 100644
index 000000000000..fb706b27f265
--- /dev/null
+++ b/changelog.d/15138.misc
@@ -0,0 +1 @@
+Fix a typo in an experimental config setting.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 54c91953e106..bc38fae0b6d1 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -175,8 +175,8 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         )
 
         # MSC3873: Disambiguate event_match keys.
-        self.msc3783_escape_event_match_key = experimental.get(
-            "msc3783_escape_event_match_key", False
+        self.msc3873_escape_event_match_key = experimental.get(
+            "msc3873_escape_event_match_key", False
         )
 
         # MSC3952: Intentional mentions, this depends on MSC3758.
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index 8f834be7742a..3c4a152d6b72 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -276,7 +276,7 @@ async def _related_events(
                 if related_event is not None:
                     related_events[relation_type] = _flatten_dict(
                         related_event,
-                        msc3783_escape_event_match_key=self.hs.config.experimental.msc3783_escape_event_match_key,
+                        msc3873_escape_event_match_key=self.hs.config.experimental.msc3873_escape_event_match_key,
                     )
 
             reply_event_id = (
@@ -294,7 +294,7 @@ async def _related_events(
                 if related_event is not None:
                     related_events["m.in_reply_to"] = _flatten_dict(
                         related_event,
-                        msc3783_escape_event_match_key=self.hs.config.experimental.msc3783_escape_event_match_key,
+                        msc3873_escape_event_match_key=self.hs.config.experimental.msc3873_escape_event_match_key,
                     )
 
                     # indicate that this is from a fallback relation.
@@ -412,7 +412,7 @@ async def _action_for_event_by_user(
         evaluator = PushRuleEvaluator(
             _flatten_dict(
                 event,
-                msc3783_escape_event_match_key=self.hs.config.experimental.msc3783_escape_event_match_key,
+                msc3873_escape_event_match_key=self.hs.config.experimental.msc3873_escape_event_match_key,
             ),
             has_mentions,
             user_mentions,
@@ -507,7 +507,7 @@ def _flatten_dict(
     prefix: Optional[List[str]] = None,
     result: Optional[Dict[str, JsonValue]] = None,
     *,
-    msc3783_escape_event_match_key: bool = False,
+    msc3873_escape_event_match_key: bool = False,
 ) -> Dict[str, JsonValue]:
     """
     Given a JSON dictionary (or event) which might contain sub dictionaries,
@@ -536,7 +536,7 @@ def _flatten_dict(
     if result is None:
         result = {}
     for key, value in d.items():
-        if msc3783_escape_event_match_key:
+        if msc3873_escape_event_match_key:
             # Escape periods in the key with a backslash (and backslashes with an
             # extra backslash). This is since a period is used as a separator between
             # nested fields.
@@ -552,7 +552,7 @@ def _flatten_dict(
                 value,
                 prefix=(prefix + [key]),
                 result=result,
-                msc3783_escape_event_match_key=msc3783_escape_event_match_key,
+                msc3873_escape_event_match_key=msc3873_escape_event_match_key,
             )
 
     # `room_version` should only ever be set when looking at the top level of an event
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index d320a12f968d..4e858fd16f33 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -54,7 +54,7 @@ def test_nested(self) -> None:
         self.assertEqual({"m.foo.b\\ar": "abc"}, _flatten_dict(input))
         self.assertEqual(
             {"m\\.foo.b\\\\ar": "abc"},
-            _flatten_dict(input, msc3783_escape_event_match_key=True),
+            _flatten_dict(input, msc3873_escape_event_match_key=True),
         )
 
     def test_non_string(self) -> None: