diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 5284940ba809..695d52066796 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -23,6 +23,7 @@
 """
 import abc
 import contextlib
+import itertools
 import logging
 from bisect import bisect
 from contextlib import contextmanager
@@ -748,14 +749,18 @@ def __init__(self, hs: "HomeServer"):
         ] = {}
 
         # Keeps track of the number of *ongoing* syncs on other processes.
+        #
         # While any sync is ongoing on another process the user will never
         # go offline.
+        #
         # Each process has a unique identifier and an update frequency. If
         # no update is received from that process within the update period then
         # we assume that all the sync requests on that process have stopped.
-        # Stored as a dict from process_id to set of user_id, and a dict of
-        # process_id to millisecond timestamp last updated.
-        self.external_process_to_current_syncs: Dict[str, Set[str]] = {}
+        # Stored as a dict from process_id to set of (user_id, device_id), and
+        # a dict of process_id to millisecond timestamp last updated.
+        self.external_process_to_current_syncs: Dict[
+            str, Set[Tuple[str, Optional[str]]]
+        ] = {}
         self.external_process_last_updated_ms: Dict[str, int] = {}
 
         self.external_sync_linearizer = Linearizer(name="external_sync_linearizer")
@@ -960,7 +965,10 @@ async def _handle_timeouts(self) -> None:
             # that were syncing on that process to see if they need to be timed
             # out.
             users_to_check.update(
-                self.external_process_to_current_syncs.pop(process_id, ())
+                user_id
+                for user_id, device_id in self.external_process_to_current_syncs.pop(
+                    process_id, ()
+                )
             )
             self.external_process_last_updated_ms.pop(process_id)
 
@@ -973,14 +981,13 @@ async def _handle_timeouts(self) -> None:
 
         syncing_user_ids = {
             user_id
-            for (
-                user_id,
-                device_id,
-            ), count in self._user_device_to_num_current_syncs.items()
+            for (user_id, _), count in self._user_device_to_num_current_syncs.items()
             if count
         }
-        for user_ids in self.external_process_to_current_syncs.values():
-            syncing_user_ids.update(user_ids)
+        for user_id, device_id in itertools.chain(
+            *self.external_process_to_current_syncs.values()
+        ):
+            syncing_user_ids.add(user_id)
 
         changes = handle_timeouts(
             states,
@@ -1106,26 +1113,27 @@ async def update_external_syncs_row(
                 process_id, set()
             )
 
-            # USER_SYNC is sent when a user starts or stops syncing on a remote
-            # process. (But only for the initial and last device.)
+            # USER_SYNC is sent when a user's device starts or stops syncing on
+            # a remote # process. (But only for the initial and last sync for that
+            # device.)
             #
-            # When a user *starts* syncing it also calls set_state(...) which
+            # When a device *starts* syncing it also calls set_state(...) which
             # will update the state, last_active_ts, and last_user_sync_ts.
-            # Simply ensure the user is tracked as syncing in this case.
+            # Simply ensure the user & device is tracked as syncing in this case.
             #
-            # When a user *stops* syncing, update the last_user_sync_ts and mark
+            # When a device *stops* syncing, update the last_user_sync_ts and mark
             # them as no longer syncing. Note this doesn't quite match the
             # monolith behaviour, which updates last_user_sync_ts at the end of
             # every sync, not just the last in-flight sync.
-            if is_syncing and user_id not in process_presence:
-                process_presence.add(user_id)
-            elif not is_syncing and user_id in process_presence:
+            if is_syncing and (user_id, device_id) not in process_presence:
+                process_presence.add((user_id, device_id))
+            elif not is_syncing and (user_id, device_id) in process_presence:
                 new_state = prev_state.copy_and_replace(
                     last_user_sync_ts=sync_time_msec
                 )
                 await self._update_states([new_state])
 
-                process_presence.discard(user_id)
+                process_presence.discard((user_id, device_id))
 
             self.external_process_last_updated_ms[process_id] = self.clock.time_msec()
 
@@ -1139,7 +1147,9 @@ async def update_external_syncs_clear(self, process_id: str) -> None:
             process_presence = self.external_process_to_current_syncs.pop(
                 process_id, set()
             )
-            prev_states = await self.current_state_for_users(process_presence)
+            prev_states = await self.current_state_for_users(
+                {user_id for user_id, device_id in process_presence}
+            )
             time_now_ms = self.clock.time_msec()
 
             await self._update_states(