diff --git a/changelog.d/16218.feature b/changelog.d/16218.feature
new file mode 100644
index 000000000000..4afd092e889a
--- /dev/null
+++ b/changelog.d/16218.feature
@@ -0,0 +1 @@
+Add `last_seen_ts` to the admin users API.
diff --git a/docs/admin_api/user_admin_api.md b/docs/admin_api/user_admin_api.md
index 8032e05497ad..975a7a0da4ab 100644
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -242,6 +242,7 @@ The following parameters should be set in the URL:
   - `displayname` - Users are ordered alphabetically by `displayname`.
   - `avatar_url` - Users are ordered alphabetically by avatar URL.
   - `creation_ts` - Users are ordered by when the users was created in ms.
+  - `last_seen_ts` - Users are ordered by when the user was lastly seen in ms.
 
 - `dir` - Direction of media order. Either `f` for forwards or `b` for backwards.
   Setting this value to `b` will reverse the above sort order. Defaults to `f`.
@@ -272,6 +273,7 @@ The following fields are returned in the JSON response body:
   - `displayname` - string - The user's display name if they have set one.
   - `avatar_url` - string -  The user's avatar URL if they have set one.
   - `creation_ts` - integer - The user's creation timestamp in ms.
+  - `last_seen_ts` - integer - The user's last activity timestamp in ms.
 
 - `next_token`: string representing a positive integer - Indication for pagination. See above.
 - `total` - integer - Total number of media.
diff --git a/synapse/handlers/admin.py b/synapse/handlers/admin.py
index 0e812a6d8b51..2f0e5f3b0a9e 100644
--- a/synapse/handlers/admin.py
+++ b/synapse/handlers/admin.py
@@ -76,6 +76,7 @@ async def get_user(self, user: UserID) -> Optional[JsonDict]:
             "consent_ts",
             "user_type",
             "is_guest",
+            "last_seen_ts",
         }
 
         if self._msc3866_enabled:
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 625a47ec1a5a..91898a5c135c 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -132,6 +132,7 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                 UserSortOrder.AVATAR_URL.value,
                 UserSortOrder.SHADOW_BANNED.value,
                 UserSortOrder.CREATION_TS.value,
+                UserSortOrder.LAST_SEEN_TS.value,
             ),
         )
 
diff --git a/synapse/storage/databases/main/__init__.py b/synapse/storage/databases/main/__init__.py
index a85633efcde8..0836e247ef5f 100644
--- a/synapse/storage/databases/main/__init__.py
+++ b/synapse/storage/databases/main/__init__.py
@@ -277,6 +277,10 @@ def get_users_paginate_txn(
                 FROM users as u
                 LEFT JOIN profiles AS p ON u.name = p.full_user_id
                 LEFT JOIN erased_users AS eu ON u.name = eu.user_id
+                LEFT JOIN (
+                    SELECT user_id, MAX(last_seen) AS last_seen_ts
+                    FROM user_ips GROUP BY user_id
+                ) ls ON u.name = ls.user_id
                 {where_clause}
                 """
             sql = "SELECT COUNT(*) as total_users " + sql_base
@@ -286,7 +290,7 @@ def get_users_paginate_txn(
             sql = f"""
                 SELECT name, user_type, is_guest, admin, deactivated, shadow_banned,
                 displayname, avatar_url, creation_ts * 1000 as creation_ts, approved,
-                eu.user_id is not null as erased
+                eu.user_id is not null as erased, last_seen_ts
                 {sql_base}
                 ORDER BY {order_by_column} {order}, u.name ASC
                 LIMIT ? OFFSET ?
diff --git a/synapse/storage/databases/main/registration.py b/synapse/storage/databases/main/registration.py
index d3a01d526fb8..7e85b73e8e3a 100644
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -206,8 +206,12 @@ def get_user_by_id_txn(txn: LoggingTransaction) -> Optional[Dict[str, Any]]:
                     consent_server_notice_sent, appservice_id, creation_ts, user_type,
                     deactivated, COALESCE(shadow_banned, FALSE) AS shadow_banned,
                     COALESCE(approved, TRUE) AS approved,
-                    COALESCE(locked, FALSE) AS locked
+                    COALESCE(locked, FALSE) AS locked, last_seen_ts
                 FROM users
+                LEFT JOIN (
+                    SELECT user_id, MAX(last_seen) AS last_seen_ts
+                    FROM user_ips GROUP BY user_id
+                ) ls ON users.name = ls.user_id
                 WHERE name = ?
                 """,
                 (user_id,),
@@ -268,6 +272,7 @@ async def get_userinfo_by_id(self, user_id: str) -> Optional[UserInfo]:
             is_shadow_banned=bool(user_data["shadow_banned"]),
             user_id=UserID.from_string(user_data["name"]),
             user_type=user_data["user_type"],
+            last_seen_ts=user_data["last_seen_ts"],
         )
 
     async def is_trial_user(self, user_id: str) -> bool:
diff --git a/synapse/storage/databases/main/stats.py b/synapse/storage/databases/main/stats.py
index 6298f0984d3c..3a2966b9e46a 100644
--- a/synapse/storage/databases/main/stats.py
+++ b/synapse/storage/databases/main/stats.py
@@ -107,6 +107,7 @@ class UserSortOrder(Enum):
     AVATAR_URL = "avatar_url"
     SHADOW_BANNED = "shadow_banned"
     CREATION_TS = "creation_ts"
+    LAST_SEEN_TS = "last_seen_ts"
 
 
 class StatsStore(StateDeltasStore):
diff --git a/synapse/types/__init__.py b/synapse/types/__init__.py
index e750417189d1..488714f60cb6 100644
--- a/synapse/types/__init__.py
+++ b/synapse/types/__init__.py
@@ -946,6 +946,7 @@ class UserInfo:
         is_guest:  True if the user is a guest user.
         is_shadow_banned:  True if the user has been shadow-banned.
         user_type:  User type (None for normal user, 'support' and 'bot' other options).
+        last_seen_ts:  Last activity timestamp of the user.
     """
 
     user_id: UserID
@@ -958,6 +959,7 @@ class UserInfo:
     is_deactivated: bool
     is_guest: bool
     is_shadow_banned: bool
+    last_seen_ts: Optional[int]
 
 
 class UserProfile(TypedDict):
diff --git a/tests/rest/admin/test_user.py b/tests/rest/admin/test_user.py
index 2f6bd0d74faa..761871b933e2 100644
--- a/tests/rest/admin/test_user.py
+++ b/tests/rest/admin/test_user.py
@@ -40,6 +40,7 @@
     user_directory,
 )
 from synapse.server import HomeServer
+from synapse.storage.databases.main.client_ips import LAST_SEEN_GRANULARITY
 from synapse.types import JsonDict, UserID, create_requester
 from synapse.util import Clock
 
@@ -456,6 +457,7 @@ class UsersListTestCase(unittest.HomeserverTestCase):
     servlets = [
         synapse.rest.admin.register_servlets,
         login.register_servlets,
+        room.register_servlets,
     ]
     url = "/_synapse/admin/v2/users"
 
@@ -506,6 +508,62 @@ def test_all_users(self) -> None:
         # Check that all fields are available
         self._check_fields(channel.json_body["users"])
 
+    def test_last_seen(self) -> None:
+        """
+        Test that last_seen_ts field is properly working.
+        """
+        user1 = self.register_user("u1", "pass")
+        user1_token = self.login("u1", "pass")
+        user2 = self.register_user("u2", "pass")
+        user2_token = self.login("u2", "pass")
+        user3 = self.register_user("u3", "pass")
+        user3_token = self.login("u3", "pass")
+
+        self.helper.create_room_as(self.admin_user, tok=self.admin_user_tok)
+        self.reactor.advance(10)
+        self.helper.create_room_as(user2, tok=user2_token)
+        self.reactor.advance(10)
+        self.helper.create_room_as(user1, tok=user1_token)
+        self.reactor.advance(10)
+        self.helper.create_room_as(user3, tok=user3_token)
+        self.reactor.advance(10)
+
+        channel = self.make_request(
+            "GET",
+            self.url,
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+        self.assertEqual(4, len(channel.json_body["users"]))
+        self.assertEqual(4, channel.json_body["total"])
+
+        admin_last_seen = channel.json_body["users"][0]["last_seen_ts"]
+        user1_last_seen = channel.json_body["users"][1]["last_seen_ts"]
+        user2_last_seen = channel.json_body["users"][2]["last_seen_ts"]
+        user3_last_seen = channel.json_body["users"][3]["last_seen_ts"]
+        self.assertTrue(admin_last_seen > 0 and admin_last_seen < 10000)
+        self.assertTrue(user2_last_seen > 10000 and user2_last_seen < 20000)
+        self.assertTrue(user1_last_seen > 20000 and user1_last_seen < 30000)
+        self.assertTrue(user3_last_seen > 30000 and user3_last_seen < 40000)
+
+        self._order_test([self.admin_user, user2, user1, user3], "last_seen_ts")
+
+        self.reactor.advance(LAST_SEEN_GRANULARITY / 1000)
+        self.helper.create_room_as(user1, tok=user1_token)
+        self.reactor.advance(10)
+
+        channel = self.make_request(
+            "GET",
+            self.url + "/" + user1,
+            access_token=self.admin_user_tok,
+        )
+        self.assertTrue(
+            channel.json_body["last_seen_ts"] > 40000 + LAST_SEEN_GRANULARITY
+        )
+
+        self._order_test([self.admin_user, user2, user3, user1], "last_seen_ts")
+
     def test_search_term(self) -> None:
         """Test that searching for a users works correctly"""
 
@@ -1135,6 +1193,7 @@ def _check_fields(self, content: List[JsonDict]) -> None:
             self.assertIn("displayname", u)
             self.assertIn("avatar_url", u)
             self.assertIn("creation_ts", u)
+            self.assertIn("last_seen_ts", u)
 
     def _create_users(self, number_users: int) -> None:
         """
@@ -3035,6 +3094,7 @@ def _check_fields(self, content: JsonDict) -> None:
         self.assertIn("consent_version", content)
         self.assertIn("consent_ts", content)
         self.assertIn("external_ids", content)
+        self.assertIn("last_seen_ts", content)
 
         # This key was removed intentionally. Ensure it is not accidentally re-included.
         self.assertNotIn("password_hash", content)
diff --git a/tests/storage/test_registration.py b/tests/storage/test_registration.py
index ba41459d083d..95c9792d546e 100644
--- a/tests/storage/test_registration.py
+++ b/tests/storage/test_registration.py
@@ -51,6 +51,7 @@ def test_register(self) -> None:
                 "locked": 0,
                 "shadow_banned": 0,
                 "approved": 1,
+                "last_seen_ts": None,
             },
             (self.get_success(self.store.get_user_by_id(self.user_id))),
         )