diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index ef6f2377cf..92c2f3b42a 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -209,6 +209,37 @@ class Ratelimiter:
 
         return allowed, time_allowed
 
+    def record_action(
+        self,
+        requester: Optional[Requester],
+        key: Optional[Hashable] = None,
+        n_actions: int = 1,
+        _time_now_s: Optional[float] = None,
+    ) -> None:
+        """Record that an action(s) took place, even if they violate the rate limit.
+
+        This is useful for tracking the frequency of events that happen across
+        federation which we still want to impose local rate limits on. For instance, if
+        we are alice.com monitoring a particular room, we cannot prevent bob.com
+        from joining users to that room. However, we can track the number of recent
+        joins in the room and refuse to serve new joins ourselves if there have been too
+        many in the room across both homeservers.
+
+        Args:
+            requester: The requester that is doing the action, if any.
+            key: An arbitrary key used to classify an action. Defaults to the
+                requester's user ID.
+            n_actions: The number of times the user wants to do this action. If the user
+                cannot do all of the actions, the user's action count is not incremented
+                at all.
+            _time_now_s: The current time. Optional, defaults to the current time according
+                to self.clock. Only used by tests.
+        """
+        key = self._get_key(requester, key)
+        time_now_s = _time_now_s if _time_now_s is not None else self.clock.time()
+        action_count, time_start, rate_hz = self._get_action_counts(key, time_now_s)
+        self.actions[key] = (action_count + n_actions, time_start, rate_hz)
+
     def _prune_message_counts(self, time_now_s: float) -> None:
         """Remove message count entries that have not exceeded their defined
         rate_hz limit
diff --git a/tests/api/test_ratelimiting.py b/tests/api/test_ratelimiting.py
index 18649c2c05..c86f783c5b 100644
--- a/tests/api/test_ratelimiting.py
+++ b/tests/api/test_ratelimiting.py
@@ -314,3 +314,77 @@ class TestRatelimiter(unittest.HomeserverTestCase):
 
         # Check that we get rate limited after using that token.
         self.assertFalse(consume_at(11.1))
+
+    def test_record_action_which_doesnt_fill_bucket(self) -> None:
+        limiter = Ratelimiter(
+            store=self.hs.get_datastores().main, clock=None, rate_hz=0.1, burst_count=3
+        )
+
+        # Observe two actions, leaving room in the bucket for one more.
+        limiter.record_action(requester=None, key="a", n_actions=2, _time_now_s=0.0)
+
+        # We should be able to take a new action now.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=0.0)
+        )
+        self.assertTrue(success)
+
+        # ... but not two.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=0.0)
+        )
+        self.assertFalse(success)
+
+    def test_record_action_which_fills_bucket(self) -> None:
+        limiter = Ratelimiter(
+            store=self.hs.get_datastores().main, clock=None, rate_hz=0.1, burst_count=3
+        )
+
+        # Observe three actions, filling up the bucket.
+        limiter.record_action(requester=None, key="a", n_actions=3, _time_now_s=0.0)
+
+        # We should be unable to take a new action now.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=0.0)
+        )
+        self.assertFalse(success)
+
+        # If we wait 10 seconds to leak a token, we should be able to take one action...
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=10.0)
+        )
+        self.assertTrue(success)
+
+        # ... but not two.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=10.0)
+        )
+        self.assertFalse(success)
+
+    def test_record_action_which_overfills_bucket(self) -> None:
+        limiter = Ratelimiter(
+            store=self.hs.get_datastores().main, clock=None, rate_hz=0.1, burst_count=3
+        )
+
+        # Observe four actions, exceeding the bucket.
+        limiter.record_action(requester=None, key="a", n_actions=4, _time_now_s=0.0)
+
+        # We should be prevented from taking a new action now.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=0.0)
+        )
+        self.assertFalse(success)
+
+        # If we wait 10 seconds to leak a token, we should be unable to take an action
+        # because the bucket is still full.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=10.0)
+        )
+        self.assertFalse(success)
+
+        # But after another 10 seconds we leak a second token, giving us room for
+        # action.
+        success, _ = self.get_success_or_raise(
+            limiter.can_do_action(requester=None, key="a", _time_now_s=20.0)
+        )
+        self.assertTrue(success)