diff --git a/changelog.d/15563.misc b/changelog.d/15563.misc
new file mode 100644
index 000000000..8bfecf2b9
--- /dev/null
+++ b/changelog.d/15563.misc
@@ -0,0 +1 @@
+Implement [MSC3821](https://github.com/matrix-org/matrix-spec-proposals/pull/3821) to update the redaction rules.
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index e65b9a028..7030b133d 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -103,6 +103,8 @@ class RoomVersion:
     msc3787_knock_restricted_join_rule: bool
     # MSC3667: Enforce integer power levels
     msc3667_int_only_power_levels: bool
+    # MSC3821: Do not redact the third_party_invite content field for membership events.
+    msc3821_redaction_rules: bool
     # MSC3931: Adds a push rule condition for "room version feature flags", making
     # some push rules room version dependent. Note that adding a flag to this list
     # is not enough to mark it "supported": the push rule evaluator also needs to
@@ -133,6 +135,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -155,6 +158,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -177,6 +181,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -199,6 +204,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -221,6 +227,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -243,6 +250,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -265,6 +273,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -287,6 +296,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -309,6 +319,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -331,6 +342,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -353,6 +365,30 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
+        msc3931_push_features=(),
+        msc3989_redaction_rules=False,
+    )
+    MSC3821 = RoomVersion(
+        "org.matrix.msc3821.opt1",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.ROOM_V4_PLUS,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
+        msc2176_redaction_rules=False,
+        msc3083_join_rules=True,
+        msc3375_redaction_rules=True,
+        msc2403_knocking=True,
+        msc2716_historical=False,
+        msc2716_redactions=False,
+        msc3389_relation_redactions=False,
+        msc3787_knock_restricted_join_rule=False,
+        msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=True,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -375,6 +411,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -397,6 +434,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=False,
     )
@@ -420,6 +458,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(PushRuleRoomFlag.EXTENSIBLE_EVENTS,),
         msc3989_redaction_rules=False,
     )
@@ -442,6 +481,7 @@ class RoomVersions:
         msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
+        msc3821_redaction_rules=False,
         msc3931_push_features=(),
         msc3989_redaction_rules=True,
     )
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index e540f1582..e6d040176 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -130,6 +130,16 @@ def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDic
         add_fields("membership")
         if room_version.msc3375_redaction_rules:
             add_fields(EventContentFields.AUTHORISING_USER)
+        if room_version.msc3821_redaction_rules:
+            # Preserve the signed field under third_party_invite.
+            third_party_invite = event_dict["content"].get("third_party_invite")
+            if isinstance(third_party_invite, collections.abc.Mapping):
+                new_content["third_party_invite"] = {}
+                if "signed" in third_party_invite:
+                    new_content["third_party_invite"]["signed"] = third_party_invite[
+                        "signed"
+                    ]
+
     elif event_type == EventTypes.Create:
         # MSC2176 rules state that create events cannot be redacted.
         if room_version.msc2176_redaction_rules:
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index 02f0800a3..e40eac2eb 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -394,7 +394,7 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
         )
 
     def test_member(self) -> None:
-        """Member events have changed behavior starting with MSC3375."""
+        """Member events have changed behavior in MSC3375 and MSC3821."""
         self.run_test(
             {
                 "type": "m.room.member",
@@ -437,6 +437,79 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
             room_version=RoomVersions.V9,
         )
 
+        # After MSC3821, the signed key under third_party_invite is protected
+        # from redaction.
+        THIRD_PARTY_INVITE = {
+            "display_name": "alice",
+            "signed": {
+                "mxid": "@alice:example.org",
+                "signatures": {
+                    "magic.forest": {
+                        "ed25519:3": "fQpGIW1Snz+pwLZu6sTy2aHy/DYWWTspTJRPyNp0PKkymfIsNffysMl6ObMMFdIJhk6g6pwlIqZ54rxo8SLmAg"
+                    }
+                },
+                "token": "abc123",
+            },
+        }
+
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": THIRD_PARTY_INVITE,
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": {"signed": THIRD_PARTY_INVITE["signed"]},
+                },
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
+        # Ensure this doesn't break if an invalid field is sent.
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": {},
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {"membership": "invite", "third_party_invite": {}},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "invite",
+                    "third_party_invite": "stripped",
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {"membership": "invite"},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.MSC3821,
+        )
+
     def test_relations(self) -> None:
         """Event relations get redacted until MSC3389."""
         # Normally the m._relates_to field is redacted.