diff --git a/changelog.d/15395.misc b/changelog.d/15395.misc
new file mode 100644
index 000000000000..ee938452416c
--- /dev/null
+++ b/changelog.d/15395.misc
@@ -0,0 +1 @@
+Implement [MSC2174](https://github.com/matrix-org/matrix-spec-proposals/pull/2174) to move the `redacts` key to a `content` property.
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index bc15f2d0631f..697c1eb47332 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -78,7 +78,8 @@ class RoomVersion:
     # MSC2209: Check 'notifications' key while verifying
     # m.room.power_levels auth rules.
     limit_notifications_power_levels: bool
-    # MSC2174/MSC2176: Apply updated redaction rules algorithm.
+    # MSC2174/MSC2176: Apply updated redaction rules algorithm, move redacts to
+    # content property.
     msc2176_redaction_rules: bool
     # MSC3083: Support the 'restricted' join_rule.
     msc3083_join_rules: bool
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index af55874b5c47..52e4c21525f8 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -786,7 +786,7 @@ def check_redaction(
     """Check whether the event sender is allowed to redact the target event.
 
     Returns:
-        True if the the sender is allowed to redact the target event if the
+        True if the sender is allowed to redact the target event if the
         target event was created by them.
         False if the sender is allowed to redact the target event with no
         further checks.
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index d475fe7ae526..4501518cf0e5 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -326,7 +326,6 @@ def __init__(
     hashes: DictProperty[Dict[str, str]] = DictProperty("hashes")
     origin: DictProperty[str] = DictProperty("origin")
     origin_server_ts: DictProperty[int] = DictProperty("origin_server_ts")
-    redacts: DefaultDictProperty[Optional[str]] = DefaultDictProperty("redacts", None)
     room_id: DictProperty[str] = DictProperty("room_id")
     sender: DictProperty[str] = DictProperty("sender")
     # TODO state_key should be Optional[str]. This is generally asserted in Synapse
@@ -346,6 +345,13 @@ def event_id(self) -> str:
     def membership(self) -> str:
         return self.content["membership"]
 
+    @property
+    def redacts(self) -> Optional[str]:
+        """MSC2176 moved the redacts field into the content."""
+        if self.room_version.msc2176_redaction_rules:
+            return self.content.get("redacts")
+        return self.get("redacts")
+
     def is_state(self) -> bool:
         return self.get_state_key() is not None
 
diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index c82745275f94..a254548c6c76 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -173,7 +173,9 @@ async def build(
         if self.is_state():
             event_dict["state_key"] = self._state_key
 
-        if self._redacts is not None:
+        # MSC2174 moves the redacts property to the content, it is invalid to
+        # provide it as a top-level property.
+        if self._redacts is not None and not self.room_version.msc2176_redaction_rules:
             event_dict["redacts"] = self._redacts
 
         if self._origin_server_ts is not None:
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index c0705d42914e..830ecbcbf4c1 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -1096,6 +1096,7 @@ def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
         self.event_creation_handler = hs.get_event_creation_handler()
         self.auth = hs.get_auth()
+        self._store = hs.get_datastores().main
         self._relation_handler = hs.get_relations_handler()
         self._msc3912_enabled = hs.config.experimental.msc3912_enabled
 
@@ -1113,6 +1114,15 @@ async def _do(
     ) -> Tuple[int, JsonDict]:
         content = parse_json_object_from_request(request)
 
+        # Ensure the redacts property in the content matches the one provided in
+        # the URL.
+        room_version = await self._store.get_room_version(room_id)
+        if room_version.msc2176_redaction_rules:
+            if "redacts" in content and content["redacts"] != event_id:
+                raise SynapseError(400, "Cannot provide", Codes.INVALID_PARAM)
+            else:
+                content["redacts"] = event_id
+
         try:
             with_relations = None
             if self._msc3912_enabled and "org.matrix.msc3912.with_relations" in content:
@@ -1128,20 +1138,23 @@ async def _do(
                     requester, txn_id, room_id
                 )
 
+            # Event is not yet redacted, create a new event to redact it.
             if event is None:
+                event_dict = {
+                    "type": EventTypes.Redaction,
+                    "content": content,
+                    "room_id": room_id,
+                    "sender": requester.user.to_string(),
+                }
+                # Earlier room versions had a top-level redacts property.
+                if not room_version.msc2176_redaction_rules:
+                    event_dict["redacts"] = event_id
+
                 (
                     event,
                     _,
                 ) = await self.event_creation_handler.create_and_send_nonmember_event(
-                    requester,
-                    {
-                        "type": EventTypes.Redaction,
-                        "content": content,
-                        "room_id": room_id,
-                        "sender": requester.user.to_string(),
-                        "redacts": event_id,
-                    },
-                    txn_id=txn_id,
+                    requester, event_dict, txn_id=txn_id
                 )
 
                 if with_relations:
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index c35f58f46213..1b179acb20fc 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -318,7 +318,11 @@ def test_redacts(self) -> None:
         """Redaction events have no special behaviour until MSC2174/MSC2176."""
 
         self.run_test(
-            {"type": "m.room.redaction", "content": {"redacts": "$test2:domain"}},
+            {
+                "type": "m.room.redaction",
+                "content": {"redacts": "$test2:domain"},
+                "redacts": "$test2:domain",
+            },
             {
                 "type": "m.room.redaction",
                 "content": {},
@@ -330,7 +334,11 @@ def test_redacts(self) -> None:
 
         # After MSC2174, redaction events keep the redacts content key.
         self.run_test(
-            {"type": "m.room.redaction", "content": {"redacts": "$test2:domain"}},
+            {
+                "type": "m.room.redaction",
+                "content": {"redacts": "$test2:domain"},
+                "redacts": "$test2:domain",
+            },
             {
                 "type": "m.room.redaction",
                 "content": {"redacts": "$test2:domain"},
diff --git a/tests/rest/client/test_redactions.py b/tests/rest/client/test_redactions.py
index 5dfe44defbb3..84a60c0b0721 100644
--- a/tests/rest/client/test_redactions.py
+++ b/tests/rest/client/test_redactions.py
@@ -16,6 +16,7 @@
 from twisted.test.proto_helpers import MemoryReactor
 
 from synapse.api.constants import EventTypes, RelationTypes
+from synapse.api.room_versions import RoomVersions
 from synapse.rest import admin
 from synapse.rest.client import login, room, sync
 from synapse.server import HomeServer
@@ -74,6 +75,7 @@ def _redact_event(
         event_id: str,
         expect_code: int = 200,
         with_relations: Optional[List[str]] = None,
+        content: Optional[JsonDict] = None,
     ) -> JsonDict:
         """Helper function to send a redaction event.
 
@@ -81,7 +83,7 @@ def _redact_event(
         """
         path = "/_matrix/client/r0/rooms/%s/redact/%s" % (room_id, event_id)
 
-        request_content = {}
+        request_content = content or {}
         if with_relations:
             request_content["org.matrix.msc3912.with_relations"] = with_relations
 
@@ -92,7 +94,7 @@ def _redact_event(
         return channel.json_body
 
     def _sync_room_timeline(self, access_token: str, room_id: str) -> List[JsonDict]:
-        channel = self.make_request("GET", "sync", access_token=self.mod_access_token)
+        channel = self.make_request("GET", "sync", access_token=access_token)
         self.assertEqual(channel.code, 200)
         room_sync = channel.json_body["rooms"]["join"][room_id]
         return room_sync["timeline"]["events"]
@@ -466,3 +468,36 @@ def test_redact_relations_txn_id_reuse(self) -> None:
         )
         self.assertIn("body", event_dict["content"], event_dict)
         self.assertEqual("I'm in a thread!", event_dict["content"]["body"])
+
+    def test_content_redaction(self) -> None:
+        """MSC2174 moved the redacts property to the content."""
+        # Create a room with the newer room version.
+        room_id = self.helper.create_room_as(
+            self.mod_user_id,
+            tok=self.mod_access_token,
+            room_version=RoomVersions.MSC2176.identifier,
+        )
+
+        # Create an event.
+        b = self.helper.send(room_id=room_id, tok=self.mod_access_token)
+        event_id = b["event_id"]
+
+        # Attempt to redact it with a bogus event ID.
+        self._redact_event(
+            self.mod_access_token,
+            room_id,
+            event_id,
+            expect_code=400,
+            content={"redacts": "foo"},
+        )
+
+        # Redact it for real.
+        self._redact_event(self.mod_access_token, room_id, event_id)
+
+        # Sync the room, to get the id of the create event
+        timeline = self._sync_room_timeline(self.mod_access_token, room_id)
+        redact_event = timeline[-1]
+        self.assertEqual(redact_event["type"], EventTypes.Redaction)
+        # The redacts key should be in the content.
+        self.assertNotIn("redacts", redact_event)
+        self.assertEquals(redact_event["content"]["redacts"], event_id)