diff --git a/crates/matrix-sdk/src/test_utils/mocks/mod.rs b/crates/matrix-sdk/src/test_utils/mocks/mod.rs
index 1a8d9a9f7..f7465fdfc 100644
--- a/crates/matrix-sdk/src/test_utils/mocks/mod.rs
+++ b/crates/matrix-sdk/src/test_utils/mocks/mod.rs
@@ -3950,6 +3950,13 @@ impl<'a> MockEndpoint<'a, PutThreadSubscriptionEndpoint> {
         self.endpoint.matchers = self.endpoint.matchers.match_thread_id(thread_root);
         self
     }
+    /// Match the request body's `automatic` field against a specific event id.
+    pub fn match_automatic_event_id(mut self, up_to_event_id: &EventId) -> Self {
+        self.mock = self.mock.and(body_json(json!({
+            "automatic": up_to_event_id
+        })));
+        self
+    }
 }
 
 /// A prebuilt mock for `DELETE
diff --git a/crates/matrix-sdk/tests/integration/event_cache/threads.rs b/crates/matrix-sdk/tests/integration/event_cache/threads.rs
index fd268baff..ccab2f3af 100644
--- a/crates/matrix-sdk/tests/integration/event_cache/threads.rs
+++ b/crates/matrix-sdk/tests/integration/event_cache/threads.rs
@@ -1,19 +1,30 @@
+use std::time::Duration;
+
 use assert_matches2::assert_let;
 use eyeball_im::VectorDiff;
 use imbl::Vector;
 use matrix_sdk::{
     assert_let_timeout,
     deserialized_responses::{ThreadSummaryStatus, TimelineEvent},
-    event_cache::{RoomEventCacheUpdate, ThreadEventCacheUpdate},
+    event_cache::{RoomEventCacheSubscriber, RoomEventCacheUpdate, ThreadEventCacheUpdate},
+    sleep::sleep,
     test_utils::{
         assert_event_matches_msg,
         mocks::{MatrixMockServer, RoomRelationsResponseTemplate},
     },
+    Client, ThreadingSupport,
 };
 use matrix_sdk_test::{
     async_test, event_factory::EventFactory, GlobalAccountDataTestEvent, JoinedRoomBuilder, ALICE,
 };
-use ruma::{event_id, room_id, user_id};
+use ruma::{
+    event_id,
+    events::{AnySyncTimelineEvent, Mentions},
+    push::{ConditionalPushRule, Ruleset},
+    room_id,
+    serde::Raw,
+    user_id, OwnedEventId, OwnedRoomId,
+};
 use serde_json::json;
 use tokio::sync::broadcast;
 
@@ -433,3 +444,344 @@ async fn test_deduplication() {
     // The events were already known, so the stream is still empty.
     assert!(thread_stream.is_empty());
 }
+
+struct ThreadSubscriptionTestSetup {
+    server: MatrixMockServer,
+    client: Client,
+    factory: EventFactory,
+    room_id: OwnedRoomId,
+    subscriber: RoomEventCacheSubscriber,
+    /// 3 events: 1 non-mention, 1 mention, and another non-mention.
+    events: Vec<Raw<AnySyncTimelineEvent>>,
+    mention_event_id: OwnedEventId,
+    thread_root: OwnedEventId,
+}
+
+/// Create a new setup for a thread subscription test, with enough data so that
+/// a push context can be created.
+///
+/// The setup uses custom push rules, to trigger notifications only on mentions.
+///
+/// The setup includes 3 events (1 non-mention, 1 mention, and another
+/// non-mention) in the same thread, for easy testing of automated
+/// subscriptions.
+async fn thread_subscription_test_setup() -> ThreadSubscriptionTestSetup {
+    let server = MatrixMockServer::new().await;
+
+    let thread_root = event_id!("$thread_root");
+
+    // Assuming a client that's interested in thread subscriptions,
+    let client = server
+        .client_builder()
+        .on_builder(|builder| {
+            builder.with_threading_support(ThreadingSupport::Enabled { with_subscriptions: true })
+        })
+        .build()
+        .await;
+
+    // Immediately subscribe the event cache to sync updates.
+    client.event_cache().subscribe().unwrap();
+
+    let room_id = room_id!("!omelette:fromage.fr");
+    let room = server.sync_joined_room(&client, room_id).await;
+
+    let (room_event_cache, _drop_handles) = room.event_cache().await.unwrap();
+
+    let (initial_events, mut subscriber) = room_event_cache.subscribe().await;
+    assert!(initial_events.is_empty());
+    assert!(subscriber.is_empty());
+
+    // Provide a dummy sync with the room's member profile of the current user, so
+    // the push context can be created.
+    let own_user_id = client.user_id().unwrap();
+    let f = EventFactory::new().room(room_id).sender(*ALICE);
+    let member = f.member(own_user_id).sender(own_user_id);
+
+    // Override push rules so that only an intentional mention causes a
+    // notification.
+    let mut push_rules = Ruleset::default();
+    push_rules.override_.insert(ConditionalPushRule::is_user_mention(own_user_id));
+
+    server
+        .mock_sync()
+        .ok_and_run(&client, |sync_builder| {
+            sync_builder.add_joined_room(JoinedRoomBuilder::new(room_id).add_state_event(member));
+            sync_builder.add_global_account_data_event(GlobalAccountDataTestEvent::Custom(json!({
+                "type": "m.push_rules",
+                "content": {
+                    "global": push_rules
+                }
+            })));
+        })
+        .await;
+
+    // Wait for the initial sync processing to complete; it will trigger a member
+    // update, at the very least.
+    assert_let_timeout!(Ok(RoomEventCacheUpdate::UpdateMembers { .. }) = subscriber.recv());
+
+    let first_reply_event_id = event_id!("$first_reply");
+    let first_reply = f
+        .text_msg("hey there")
+        .in_thread(thread_root, thread_root)
+        .event_id(first_reply_event_id)
+        .into_raw();
+
+    let second_reply_event_id = event_id!("$second_reply");
+    let second_reply = f
+        .text_msg("hoy test user!")
+        .mentions(Mentions::with_user_ids([own_user_id.to_owned()]))
+        .in_thread(thread_root, first_reply_event_id)
+        .event_id(second_reply_event_id)
+        .into_raw();
+
+    let third_reply_event_id = event_id!("$third_reply");
+    let third_reply = f
+        .text_msg("ciao!")
+        .in_thread(thread_root, second_reply_event_id)
+        .event_id(third_reply_event_id)
+        .into_raw();
+
+    ThreadSubscriptionTestSetup {
+        server,
+        client,
+        factory: f,
+        subscriber,
+        events: vec![first_reply, second_reply, third_reply],
+        mention_event_id: second_reply_event_id.to_owned(),
+        thread_root: thread_root.to_owned(),
+        room_id: room_id.to_owned(),
+    }
+}
+
+#[async_test]
+async fn test_auto_subscribe_thread_via_sync() {
+    let mut s = thread_subscription_test_setup().await;
+
+    // (The endpoint will be called for the current thread, and with an automatic
+    // subscription up to the given event ID.)
+    s.server
+        .mock_put_thread_subscription()
+        .match_automatic_event_id(&s.mention_event_id)
+        .match_thread_id(s.thread_root.to_owned())
+        .ok()
+        .mock_once()
+        .mount()
+        .await;
+
+    let mut thread_subscriber_updates =
+        s.client.event_cache().subscribe_thread_subscriber_updates();
+
+    // When I receive 3 events (1 non mention, 1 mention, then 1 non mention again),
+    // from sync, I'll get subscribed to the thread because of the second event.
+    s.server
+        .sync_room(&s.client, JoinedRoomBuilder::new(&s.room_id).add_timeline_bulk(s.events))
+        .await;
+
+    // Let the event cache process the update.
+    assert_let_timeout!(
+        Ok(RoomEventCacheUpdate::UpdateTimelineEvents { .. }) = s.subscriber.recv()
+    );
+    assert_let_timeout!(Ok(()) = thread_subscriber_updates.recv());
+
+    // The actual check is the `mock_once` call above!
+}
+
+#[async_test]
+async fn test_dont_auto_subscribe_on_already_subscribed_thread() {
+    let mut s = thread_subscription_test_setup().await;
+
+    // Given a thread I'm already subscribed to,
+    s.server
+        .mock_get_thread_subscription()
+        .match_thread_id(s.thread_root.to_owned())
+        .ok(false)
+        .mock_once()
+        .mount()
+        .await;
+
+    // The PUT endpoint (to subscribe to the thread) shouldn't be called…
+    s.server.mock_put_thread_subscription().ok().expect(0).mount().await;
+
+    // …when I receive a new in-thread mention for this thread.
+    s.server
+        .sync_room(&s.client, JoinedRoomBuilder::new(&s.room_id).add_timeline_bulk(s.events))
+        .await;
+
+    // Let the event cache process the update.
+    assert_let_timeout!(
+        Ok(RoomEventCacheUpdate::UpdateTimelineEvents { .. }) = s.subscriber.recv()
+    );
+
+    // Let a bit of time for the background thread subscriber task to process the
+    // update.
+    sleep(Duration::from_millis(200)).await;
+
+    // The actual check is the `expect` call above!
+}
+
+#[async_test]
+async fn test_auto_subscribe_on_thread_paginate() {
+    // In this scenario, we're back-paginating a thread and making sure that the
+    // back-paginated events do cause a subscription.
+
+    let s = thread_subscription_test_setup().await;
+
+    let event_cache = s.client.event_cache();
+    event_cache.subscribe().unwrap();
+
+    let mut thread_subscriber_updates =
+        s.client.event_cache().subscribe_thread_subscriber_updates();
+
+    let thread_root_id = event_id!("$thread_root");
+    let thread_resp_id = event_id!("$thread_resp");
+
+    // Receive an in-thread event.
+    let room = s
+        .server
+        .sync_room(
+            &s.client,
+            JoinedRoomBuilder::new(&s.room_id).add_timeline_event(
+                s.factory
+                    .text_msg("that's a good point")
+                    .in_thread(thread_root_id, thread_root_id)
+                    .event_id(thread_resp_id),
+            ),
+        )
+        .await;
+
+    let (room_event_cache, _drop_handles) = room.event_cache().await.unwrap();
+
+    let (thread_events, mut thread_stream) =
+        room_event_cache.subscribe_to_thread(thread_root_id.to_owned()).await;
+
+    // Sanity check: the sync event is added to the thread.
+    let mut thread_events = wait_for_initial_events(thread_events, &mut thread_stream).await;
+    assert_eq!(thread_events.len(), 1);
+    assert_eq!(thread_events.remove(0).event_id().as_deref(), Some(thread_resp_id));
+
+    assert!(thread_subscriber_updates.is_empty());
+
+    // It's possible to paginate the thread, and this will push the thread root
+    // because there's no prev-batch token.
+    let reversed_events = s.events.into_iter().rev().map(Raw::cast_unchecked).collect();
+    s.server
+        .mock_room_relations()
+        .match_target_event(thread_root_id.to_owned())
+        .ok(RoomRelationsResponseTemplate::default().events(reversed_events))
+        .mock_once()
+        .mount()
+        .await;
+
+    s.server
+        .mock_room_event()
+        .match_event_id()
+        .ok(s.factory.text_msg("Thread root").event_id(thread_root_id).into())
+        .mock_once()
+        .mount()
+        .await;
+
+    // (The endpoint will be called for the current thread, and with an automatic
+    // subscription up to the given event ID.)
+    s.server
+        .mock_put_thread_subscription()
+        .match_automatic_event_id(&s.mention_event_id)
+        .match_thread_id(s.thread_root.to_owned())
+        .ok()
+        .mock_once()
+        .mount()
+        .await;
+
+    let hit_start =
+        room_event_cache.paginate_thread_backwards(thread_root_id.to_owned(), 42).await.unwrap();
+    assert!(hit_start);
+
+    // Let the event cache process the update.
+    assert_let_timeout!(Ok(ThreadEventCacheUpdate { .. }) = thread_stream.recv());
+    assert_let_timeout!(Ok(()) = thread_subscriber_updates.recv());
+    assert!(thread_subscriber_updates.is_empty());
+}
+
+#[async_test]
+async fn test_auto_subscribe_on_thread_paginate_root_event() {
+    // In this scenario, the root of a thread is the event that would cause the
+    // subscription.
+
+    let s = thread_subscription_test_setup().await;
+
+    let event_cache = s.client.event_cache();
+    event_cache.subscribe().unwrap();
+
+    let mut thread_subscriber_updates =
+        s.client.event_cache().subscribe_thread_subscriber_updates();
+
+    let thread_root_id = event_id!("$thread_root");
+    let thread_resp_id = event_id!("$thread_resp");
+
+    // Receive an in-thread event.
+    let room = s
+        .server
+        .sync_room(
+            &s.client,
+            JoinedRoomBuilder::new(&s.room_id).add_timeline_event(
+                s.factory
+                    .text_msg("that's a good point")
+                    .in_thread(thread_root_id, thread_root_id)
+                    .event_id(thread_resp_id),
+            ),
+        )
+        .await;
+
+    let (room_event_cache, _drop_handles) = room.event_cache().await.unwrap();
+
+    let (thread_events, mut thread_stream) =
+        room_event_cache.subscribe_to_thread(thread_root_id.to_owned()).await;
+
+    // Sanity check: the sync event is added to the thread.
+    let mut thread_events = wait_for_initial_events(thread_events, &mut thread_stream).await;
+    assert_eq!(thread_events.len(), 1);
+    assert_eq!(thread_events.remove(0).event_id().as_deref(), Some(thread_resp_id));
+
+    assert!(thread_subscriber_updates.is_empty());
+
+    // It's possible to paginate the thread, and this will push the thread root
+    // because there's no prev-batch token.
+    s.server
+        .mock_room_relations()
+        .match_target_event(thread_root_id.to_owned())
+        .ok(RoomRelationsResponseTemplate::default())
+        .mock_once()
+        .mount()
+        .await;
+
+    s.server
+        .mock_room_event()
+        .match_event_id()
+        .ok(s
+            .factory
+            .text_msg("da r00t")
+            .event_id(thread_root_id)
+            .mentions(Mentions::with_user_ids(s.client.user_id().map(ToOwned::to_owned)))
+            .into())
+        .mock_once()
+        .mount()
+        .await;
+
+    // (The endpoint will be called for the current thread, and with an automatic
+    // subscription up to the given event ID.)
+    s.server
+        .mock_put_thread_subscription()
+        .match_automatic_event_id(thread_root_id)
+        .match_thread_id(thread_root_id.to_owned())
+        .ok()
+        .mock_once()
+        .mount()
+        .await;
+
+    let hit_start =
+        room_event_cache.paginate_thread_backwards(thread_root_id.to_owned(), 42).await.unwrap();
+    assert!(hit_start);
+
+    // Let the event cache process the update.
+    assert_let_timeout!(Ok(ThreadEventCacheUpdate { .. }) = thread_stream.recv());
+    assert_let_timeout!(Ok(()) = thread_subscriber_updates.recv());
+}