diff --git a/changelog.d/7736.feature b/changelog.d/7736.feature
new file mode 100644
index 000000000000..c97864677aac
--- /dev/null
+++ b/changelog.d/7736.feature
@@ -0,0 +1 @@
+Add unread messages count to sync responses.
diff --git a/synapse/storage/data_stores/main/events.py b/synapse/storage/data_stores/main/events.py
index cfd24d2f061d..184038556fad 100644
--- a/synapse/storage/data_stores/main/events.py
+++ b/synapse/storage/data_stores/main/events.py
@@ -25,6 +25,7 @@
 from canonicaljson import json
 from prometheus_client import Counter
 
+from twisted.enterprise.adbapi import Connection
 from twisted.internet import defer
 
 import synapse.metrics
@@ -61,6 +62,12 @@
     ["type", "origin_type", "origin_entity"],
 )
 
+STATE_EVENT_TYPES_TO_MARK_UNREAD = [
+    EventTypes.PowerLevels,
+    EventTypes.Topic,
+    EventTypes.Name,
+]
+
 
 def encode_json(json_object):
     """
@@ -977,7 +984,7 @@ def _update_metadata_tables_txn(
             txn, events=[event for event, _ in events_and_contexts]
         )
 
-        for event, _ in events_and_contexts:
+        for event, context in events_and_contexts:
             if event.type == EventTypes.Name:
                 # Insert into the event_search table.
                 self._store_room_name_txn(txn, event)
@@ -1009,6 +1016,8 @@ def _update_metadata_tables_txn(
                 if isinstance(expiry_ts, int) and not event.is_state():
                     self._insert_event_expiry_txn(txn, event.event_id, expiry_ts)
 
+            self._maybe_insert_unread_event_txn(txn, event, context)
+
         # Insert into the room_memberships table.
         self._store_room_members_txn(
             txn,
@@ -1614,3 +1623,72 @@ def f(txn, stream_ordering):
             await self.db.runInteraction("locally_reject_invite", f, stream_ordering)
 
         return stream_ordering
+
+    def _maybe_insert_unread_event_txn(
+        self, txn: Connection, event: EventBase, context: EventContext,
+    ):
+        """Mark the event as unread for every current member of the room if it passes the
+        conditions for that.
+
+        These conditions are: the event must either have a body, be an encrypted message,
+        or be either a power levels event, a room name event or a room topic event, and
+        must be neither rejected or soft-failed nor an edit or a notice.
+
+        Args:
+            txn: The transaction to use to retrieve room members and to mark the event
+                as unread.
+            event: The event to evaluate and maybe mark as unread.
+            context: The context in which the event was sent (used to figure out whether
+                the event has been rejected).
+        """
+        content = event.content
+
+        is_edit = (
+            content.get("m.relates_to", {}).get("rel_type") == RelationTypes.REPLACE
+        )
+        is_notice = not event.is_state() and content.get("msgtype") == "m.notice"
+
+        # We don't want rejected or soft-failed events, edits or notices to be marked
+        # unread.
+        if (
+            context.rejected
+            or is_edit
+            or is_notice
+            or event.internal_metadata.is_soft_failed()
+        ):
+            return
+
+        body_exists = content.get("body") is not None
+        is_state_event_to_mark_unread = (
+            event.is_state() and event.type in STATE_EVENT_TYPES_TO_MARK_UNREAD
+        )
+        is_encrypted_message = (
+            not event.is_state() and event.type == EventTypes.Encrypted
+        )
+
+        # We want to mark unread messages with a body, some state events (power levels,
+        # room name, room topic) and encrypted messages.
+        if not (body_exists or is_state_event_to_mark_unread or is_encrypted_message):
+            return
+
+        # Get the list of users that are currently joined to the room.
+        users_in_room = self.db.simple_select_onecol_txn(
+            txn=txn,
+            table="room_memberships",
+            keyvalues={"membership": Membership.JOIN, "room_id": event.room_id},
+            retcol="user_id",
+        )
+
+        # Mark the message as unread for every user currently in the room.
+        self.db.simple_insert_many_txn(
+            txn=txn,
+            table="unread_messages",
+            values=[
+                {
+                    "user_id": user_id,
+                    "stream_ordering": event.internal_metadata.stream_ordering,
+                    "room_id": event.room_id,
+                }
+                for user_id in users_in_room
+            ],
+        )
diff --git a/synapse/storage/data_stores/main/schema/delta/58/09unread_messages.sql b/synapse/storage/data_stores/main/schema/delta/58/09unread_messages.sql
new file mode 100644
index 000000000000..3c7d83b9bbd4
--- /dev/null
+++ b/synapse/storage/data_stores/main/schema/delta/58/09unread_messages.sql
@@ -0,0 +1,21 @@
+/* Copyright 2020 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+CREATE TABLE IF NOT EXISTS unread_messages(
+    user_id TEXT NOT NULL,  -- The user for which the message is unread.
+    stream_ordering BIGINT NOT NULL,  -- The position of the message in the event stream.
+    room_id TEXT NOT NULL,
+    UNIQUE (user_id, stream_ordering)
+);
\ No newline at end of file