diff --git a/changelog.d/13202.feature b/changelog.d/13202.feature
new file mode 100644
index 000000000000..d0cb902dffd0
--- /dev/null
+++ b/changelog.d/13202.feature
@@ -0,0 +1 @@
+Experimental support for thread-specific receipts ([MSC3771](https://github.com/matrix-org/matrix-spec-proposals/pull/3771)).
diff --git a/synapse/handlers/receipts.py b/synapse/handlers/receipts.py
index 43d2882b0aa2..45f0983c6b0a 100644
--- a/synapse/handlers/receipts.py
+++ b/synapse/handlers/receipts.py
@@ -97,6 +97,7 @@ async def _received_remote_receipt(self, origin: str, content: JsonDict) -> None
                             receipt_type=receipt_type,
                             user_id=user_id,
                             event_ids=user_values["event_ids"],
+                            thread_id=None,  # TODO
                             data=user_values.get("data", {}),
                         )
                     )
@@ -114,6 +115,7 @@ async def _handle_new_receipts(self, receipts: List[ReadReceipt]) -> bool:
                 receipt.receipt_type,
                 receipt.user_id,
                 receipt.event_ids,
+                receipt.thread_id,
                 receipt.data,
             )
 
@@ -146,7 +148,12 @@ async def _handle_new_receipts(self, receipts: List[ReadReceipt]) -> bool:
         return True
 
     async def received_client_receipt(
-        self, room_id: str, receipt_type: str, user_id: str, event_id: str
+        self,
+        room_id: str,
+        receipt_type: str,
+        user_id: str,
+        event_id: str,
+        thread_id: Optional[str],
     ) -> None:
         """Called when a client tells us a local user has read up to the given
         event_id in the room.
@@ -156,6 +163,7 @@ async def received_client_receipt(
             receipt_type=receipt_type,
             user_id=user_id,
             event_ids=[event_id],
+            thread_id=thread_id,
             data={"ts": int(self.clock.time_msec())},
         )
 
diff --git a/synapse/replication/tcp/client.py b/synapse/replication/tcp/client.py
index 2f59245058e7..977538741392 100644
--- a/synapse/replication/tcp/client.py
+++ b/synapse/replication/tcp/client.py
@@ -408,7 +408,8 @@ async def _on_new_receipts(
                 receipt.receipt_type,
                 receipt.user_id,
                 [receipt.event_id],
-                receipt.data,
+                thread_id=None,  # TODO
+                data=receipt.data,
             )
             await self.federation_sender.send_read_receipt(receipt_info)
 
diff --git a/synapse/rest/client/read_marker.py b/synapse/rest/client/read_marker.py
index 8896f2df50c3..d170cf1560d8 100644
--- a/synapse/rest/client/read_marker.py
+++ b/synapse/rest/client/read_marker.py
@@ -81,6 +81,7 @@ async def on_POST(
                     receipt_type,
                     user_id=requester.user.to_string(),
                     event_id=event_id,
+                    thread_id=None,  # TODO
                 )
 
         return 200, {}
diff --git a/synapse/rest/client/receipts.py b/synapse/rest/client/receipts.py
index 409bfd43c11c..3e4aea065a56 100644
--- a/synapse/rest/client/receipts.py
+++ b/synapse/rest/client/receipts.py
@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Tuple
+from typing import TYPE_CHECKING, Optional, Tuple
 
 from synapse.api.constants import ReceiptTypes
 from synapse.api.errors import SynapseError
@@ -34,7 +34,8 @@ class ReceiptRestServlet(RestServlet):
     PATTERNS = client_patterns(
         "/rooms/(?P<room_id>[^/]*)"
         "/receipt/(?P<receipt_type>[^/]*)"
-        "/(?P<event_id>[^/]*)$"
+        "/(?P<event_id>[^/]*)"
+        "(/(?P<thread_id>[^/]*))?$"
     )
 
     def __init__(self, hs: "HomeServer"):
@@ -51,7 +52,12 @@ def __init__(self, hs: "HomeServer"):
             )
 
     async def on_POST(
-        self, request: SynapseRequest, room_id: str, receipt_type: str, event_id: str
+        self,
+        request: SynapseRequest,
+        room_id: str,
+        receipt_type: str,
+        event_id: str,
+        thread_id: Optional[str],
     ) -> Tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request)
 
@@ -77,6 +83,7 @@ async def on_POST(
                 receipt_type,
                 user_id=requester.user.to_string(),
                 event_id=event_id,
+                thread_id=thread_id,
             )
 
         return 200, {}
diff --git a/synapse/storage/databases/main/receipts.py b/synapse/storage/databases/main/receipts.py
index 0090c9f22512..757d3ceb4fa5 100644
--- a/synapse/storage/databases/main/receipts.py
+++ b/synapse/storage/databases/main/receipts.py
@@ -613,6 +613,7 @@ def _insert_linearized_receipt_txn(
         receipt_type: str,
         user_id: str,
         event_id: str,
+        thread_id: Optional[str],
         data: JsonDict,
         stream_id: int,
     ) -> Optional[int]:
@@ -636,15 +637,18 @@ def _insert_linearized_receipt_txn(
         stream_ordering = int(res["stream_ordering"]) if res else None
         rx_ts = res["received_ts"] if res else 0
 
+        # Convert None to a blank string.
+        thread_id = thread_id or ""
+
         # We don't want to clobber receipts for more recent events, so we
         # have to compare orderings of existing receipts
         if stream_ordering is not None:
             sql = (
                 "SELECT stream_ordering, event_id FROM events"
-                " INNER JOIN receipts_linearized AS r USING (event_id, room_id)"
-                " WHERE r.room_id = ? AND r.receipt_type = ? AND r.user_id = ?"
+                " INNER JOIN receipts_linearized as r USING (event_id, room_id)"
+                " WHERE r.room_id = ? AND r.receipt_type = ? AND r.user_id = ? AND r.thread_id = ?"
             )
-            txn.execute(sql, (room_id, receipt_type, user_id))
+            txn.execute(sql, (room_id, receipt_type, user_id, thread_id))
 
             for so, eid in txn:
                 if int(so) >= stream_ordering:
@@ -656,6 +660,7 @@ def _insert_linearized_receipt_txn(
                     )
                     return None
 
+        # TODO
         txn.call_after(
             self.invalidate_caches_for_receipt, room_id, receipt_type, user_id
         )
@@ -671,6 +676,7 @@ def _insert_linearized_receipt_txn(
                 "room_id": room_id,
                 "receipt_type": receipt_type,
                 "user_id": user_id,
+                "thread_id": thread_id,
             },
             values={
                 "stream_id": stream_id,
@@ -678,7 +684,7 @@ def _insert_linearized_receipt_txn(
                 "data": json_encoder.encode(data),
             },
             # receipts_linearized has a unique constraint on
-            # (user_id, room_id, receipt_type), so no need to lock
+            # (user_id, room_id, receipt_type, key), so no need to lock
             lock=False,
         )
 
@@ -728,6 +734,7 @@ async def insert_receipt(
         receipt_type: str,
         user_id: str,
         event_ids: List[str],
+        thread_id: Optional[str],
         data: dict,
     ) -> Optional[Tuple[int, int]]:
         """Insert a receipt, either from local client or remote server.
@@ -760,6 +767,7 @@ async def insert_receipt(
                 receipt_type,
                 user_id,
                 linearized_event_id,
+                thread_id,
                 data,
                 stream_id=stream_id,
                 # Read committed is actually beneficial here because we check for a receipt with
@@ -774,7 +782,8 @@ async def insert_receipt(
 
         now = self._clock.time_msec()
         logger.debug(
-            "RR for event %s in %s (%i ms old)",
+            "Receipt %s for event %s in %s (%i ms old)",
+            receipt_type,
             linearized_event_id,
             room_id,
             now - event_ts,
@@ -787,6 +796,7 @@ async def insert_receipt(
             receipt_type,
             user_id,
             event_ids,
+            thread_id,
             data,
         )
 
@@ -801,6 +811,7 @@ def _insert_graph_receipt_txn(
         receipt_type: str,
         user_id: str,
         event_ids: List[str],
+        thread_id: Optional[str],
         data: JsonDict,
     ) -> None:
         assert self._can_write_to_receipts
@@ -812,6 +823,9 @@ def _insert_graph_receipt_txn(
         # FIXME: This shouldn't invalidate the whole cache
         txn.call_after(self._get_linearized_receipts_for_room.invalidate, (room_id,))
 
+        # Convert None to a blank string.
+        thread_id = thread_id or ""
+
         self.db_pool.simple_delete_txn(
             txn,
             table="receipts_graph",
@@ -819,6 +833,7 @@ def _insert_graph_receipt_txn(
                 "room_id": room_id,
                 "receipt_type": receipt_type,
                 "user_id": user_id,
+                "thread_id": thread_id,
             },
         )
         self.db_pool.simple_insert_txn(
@@ -829,6 +844,7 @@ def _insert_graph_receipt_txn(
                 "receipt_type": receipt_type,
                 "user_id": user_id,
                 "event_ids": json_encoder.encode(event_ids),
+                "thread_id": thread_id,
                 "data": json_encoder.encode(data),
             },
         )
diff --git a/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.postgres b/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.postgres
new file mode 100644
index 000000000000..e26241fa89b6
--- /dev/null
+++ b/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.postgres
@@ -0,0 +1,25 @@
+/* Copyright 2022 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.
+ */
+
+-- Allow multiple receipts per user per room via a nullable thread_id column.
+ALTER TABLE receipts_linearized ADD COLUMN thread_id TEXT NOT NULL DEFAULT '';
+ALTER TABLE receipts_graph ADD COLUMN thread_id TEXT NOT NULL DEFAULT '';
+
+-- Rebuild the unique constraint with the thread_id.
+ALTER TABLE receipts_linearized DROP CONSTRAINT IF EXISTS receipts_linearized_uniqueness;
+ALTER TABLE receipts_linearized ADD CONSTRAINT receipts_linearized_uniqueness UNIQUE (room_id, receipt_type, user_id, thread_id);
+
+ALTER TABLE receipts_graph DROP CONSTRAINT IF EXISTS receipts_graph_uniqueness;
+ALTER TABLE receipts_graph ADD CONSTRAINT receipts_graph_uniqueness UNIQUE (room_id, receipt_type, user_id, thread_id);
diff --git a/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.sqlite b/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.sqlite
new file mode 100644
index 000000000000..914cff50b0a9
--- /dev/null
+++ b/synapse/storage/schema/main/delta/72/03_add_receipts_key.sql.sqlite
@@ -0,0 +1,67 @@
+/* Copyright 2022 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.
+ */
+
+-- Allow multiple receipts per user per room via a nullable thread_id column.
+--
+-- SQLite doesn't support modifying constraints to an existing table, so it must
+-- be recreated.
+
+-- Create the new tables.
+CREATE TABLE receipts_graph_new (
+    room_id TEXT NOT NULL,
+    receipt_type TEXT NOT NULL,
+    user_id TEXT NOT NULL,
+    event_ids TEXT NOT NULL,
+    thread_id TEXT NOT NULL DEFAULT '',
+    data TEXT NOT NULL,
+    CONSTRAINT receipts_graph_uniqueness UNIQUE (room_id, receipt_type, user_id, thread_id)
+);
+
+CREATE TABLE receipts_linearized_new (
+    stream_id BIGINT NOT NULL,
+    room_id TEXT NOT NULL,
+    receipt_type TEXT NOT NULL,
+    user_id TEXT NOT NULL,
+    event_id TEXT NOT NULL,
+    thread_id TEXT NOT NULL DEFAULT '',
+    data TEXT NOT NULL,
+    CONSTRAINT receipts_linearized_uniqueness UNIQUE (room_id, receipt_type, user_id, thread_id)
+);
+
+-- Drop the old indexes.
+DROP INDEX IF EXISTS receipts_linearized_id;
+DROP INDEX IF EXISTS receipts_linearized_room_stream;
+DROP INDEX IF EXISTS receipts_linearized_user;
+
+-- Copy the data.
+INSERT INTO receipts_graph_new (room_id, receipt_type, user_id, event_ids, data)
+    SELECT room_id, receipt_type, user_id, event_ids, data
+    FROM receipts_graph;
+INSERT INTO receipts_linearized_new (stream_id, room_id, receipt_type, user_id, event_id, data)
+    SELECT stream_id, room_id, receipt_type, user_id, event_id, data
+    FROM receipts_linearized;
+
+-- Drop the old tables.
+DROP TABLE receipts_graph;
+DROP TABLE receipts_linearized;
+
+-- Rename the tables.
+ALTER TABLE receipts_graph_new RENAME TO receipts_graph;
+ALTER TABLE receipts_linearized_new RENAME TO receipts_linearized;
+
+-- Create the indices.
+CREATE INDEX receipts_linearized_id ON receipts_linearized( stream_id );
+CREATE INDEX receipts_linearized_room_stream ON receipts_linearized( room_id, stream_id );
+CREATE INDEX receipts_linearized_user ON receipts_linearized( user_id );
diff --git a/synapse/types.py b/synapse/types.py
index 668d48d646ae..7ad483c17aa9 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -830,6 +830,7 @@ class ReadReceipt:
     receipt_type: str
     user_id: str
     event_ids: List[str]
+    thread_id: Optional[str]
     data: JsonDict
 
 
diff --git a/tests/federation/test_federation_sender.py b/tests/federation/test_federation_sender.py
index 01a1db611538..820f6c904da6 100644
--- a/tests/federation/test_federation_sender.py
+++ b/tests/federation/test_federation_sender.py
@@ -49,7 +49,12 @@ def test_send_receipts(self):
 
         sender = self.hs.get_federation_sender()
         receipt = ReadReceipt(
-            "room_id", "m.read", "user_id", ["event_id"], {"ts": 1234}
+            "room_id",
+            "m.read",
+            "user_id",
+            ["event_id"],
+            thread_id=None,
+            data={"ts": 1234},
         )
         self.successResultOf(defer.ensureDeferred(sender.send_read_receipt(receipt)))
 
@@ -89,7 +94,12 @@ def test_send_receipts_with_backoff(self):
 
         sender = self.hs.get_federation_sender()
         receipt = ReadReceipt(
-            "room_id", "m.read", "user_id", ["event_id"], {"ts": 1234}
+            "room_id",
+            "m.read",
+            "user_id",
+            ["event_id"],
+            thread_id=None,
+            data={"ts": 1234},
         )
         self.successResultOf(defer.ensureDeferred(sender.send_read_receipt(receipt)))
 
@@ -121,7 +131,12 @@ def test_send_receipts_with_backoff(self):
 
         # send the second RR
         receipt = ReadReceipt(
-            "room_id", "m.read", "user_id", ["other_id"], {"ts": 1234}
+            "room_id",
+            "m.read",
+            "user_id",
+            ["other_id"],
+            thread_id=None,
+            data={"ts": 1234},
         )
         self.successResultOf(defer.ensureDeferred(sender.send_read_receipt(receipt)))
         self.pump()
diff --git a/tests/handlers/test_appservice.py b/tests/handlers/test_appservice.py
index d96d5aa1385e..fae35896c9ee 100644
--- a/tests/handlers/test_appservice.py
+++ b/tests/handlers/test_appservice.py
@@ -447,6 +447,7 @@ def test_sending_read_receipt_batches_to_application_services(self):
                     receipt_type="m.read",
                     user_id=self.local_user,
                     event_ids=[f"$eventid_{i}"],
+                    thread_id=None,
                     data={},
                 )
             )
diff --git a/tests/replication/tcp/streams/test_receipts.py b/tests/replication/tcp/streams/test_receipts.py
index eb0011784518..ff4ff4d5f080 100644
--- a/tests/replication/tcp/streams/test_receipts.py
+++ b/tests/replication/tcp/streams/test_receipts.py
@@ -33,7 +33,12 @@ def test_receipt(self):
         # tell the master to send a new receipt
         self.get_success(
             self.hs.get_datastores().main.insert_receipt(
-                "!room:blue", "m.read", USER_ID, ["$event:blue"], {"a": 1}
+                "!room:blue",
+                "m.read",
+                USER_ID,
+                ["$event:blue"],
+                thread_id=None,
+                data={"a": 1},
             )
         )
         self.replicate()
@@ -57,7 +62,12 @@ def test_receipt(self):
 
         self.get_success(
             self.hs.get_datastores().main.insert_receipt(
-                "!room2:blue", "m.read", USER_ID, ["$event2:foo"], {"a": 2}
+                "!room2:blue",
+                "m.read",
+                USER_ID,
+                ["$event2:foo"],
+                thread_id=None,
+                data={"a": 2},
             )
         )
         self.replicate()
diff --git a/tests/storage/test_event_push_actions.py b/tests/storage/test_event_push_actions.py
index e8c53f16d9c5..6c34a561623e 100644
--- a/tests/storage/test_event_push_actions.py
+++ b/tests/storage/test_event_push_actions.py
@@ -141,6 +141,7 @@ def _mark_read(stream: int, depth: int) -> None:
                     "m.read",
                     user_id=user_id,
                     event_ids=[f"$test{stream}:example.com"],
+                    thread_id=None,
                     data={},
                 )
             )