diff --git a/changelog.d/10747.feature b/changelog.d/10747.feature
new file mode 100644
index 000000000..c1cca9a17
--- /dev/null
+++ b/changelog.d/10747.feature
@@ -0,0 +1 @@
+Support room version 9 from [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375).
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index 8abcdfd4f..a19be6707 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -70,6 +70,9 @@ class RoomVersion:
     msc2176_redaction_rules = attr.ib(type=bool)
     # MSC3083: Support the 'restricted' join_rule.
     msc3083_join_rules = attr.ib(type=bool)
+    # MSC3375: Support for the proper redaction rules for MSC3083. This mustn't
+    #          be enabled if MSC3083 is not.
+    msc3375_redaction_rules = attr.ib(type=bool)
     # MSC2403: Allows join_rules to be set to 'knock', changes auth rules to allow sending
     # m.room.membership event with membership 'knock'.
     msc2403_knocking = attr.ib(type=bool)
@@ -92,6 +95,7 @@ class RoomVersions:
         limit_notifications_power_levels=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -107,6 +111,7 @@ class RoomVersions:
         limit_notifications_power_levels=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -122,6 +127,7 @@ class RoomVersions:
         limit_notifications_power_levels=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -137,6 +143,7 @@ class RoomVersions:
         limit_notifications_power_levels=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -152,6 +159,7 @@ class RoomVersions:
         limit_notifications_power_levels=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -167,6 +175,7 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -182,6 +191,7 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=True,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -197,6 +207,7 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -212,6 +223,23 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
+        msc3375_redaction_rules=False,
+        msc2403_knocking=True,
+        msc2716_historical=False,
+        msc2716_redactions=False,
+    )
+    V9 = RoomVersion(
+        "9",
+        RoomDisposition.STABLE,
+        EventFormatVersions.V3,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        msc2176_redaction_rules=False,
+        msc3083_join_rules=True,
+        msc3375_redaction_rules=True,
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
@@ -227,6 +255,7 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=True,
         msc2716_historical=True,
         msc2716_redactions=False,
@@ -242,6 +271,7 @@ class RoomVersions:
         limit_notifications_power_levels=True,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
         msc2403_knocking=True,
         msc2716_historical=True,
         msc2716_redactions=True,
@@ -261,6 +291,7 @@ KNOWN_ROOM_VERSIONS: Dict[str, RoomVersion] = {
         RoomVersions.V7,
         RoomVersions.MSC2716,
         RoomVersions.V8,
+        RoomVersions.V9,
     )
 }
 
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 738a151ce..fb22337e2 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -104,6 +104,8 @@ def prune_event_dict(room_version: RoomVersion, event_dict: dict) -> dict:
 
     if event_type == EventTypes.Member:
         add_fields("membership")
+        if room_version.msc3375_redaction_rules:
+            add_fields("join_authorised_via_users_server")
     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 7a826c086..5446fda5e 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -322,7 +322,7 @@ class PruneEventTestCase(unittest.TestCase):
             },
         )
 
-        # After MSC3083, alias events have no special behavior.
+        # After MSC3083, the allow key is protected from redaction.
         self.run_test(
             {
                 "type": "m.room.join_rules",
@@ -344,6 +344,50 @@ class PruneEventTestCase(unittest.TestCase):
             room_version=RoomVersions.V8,
         )
 
+    def test_member(self):
+        """Member events have changed behavior starting with MSC3375."""
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "event_id": "$test:domain",
+                "content": {
+                    "membership": "join",
+                    "join_authorised_via_users_server": "@user:domain",
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "event_id": "$test:domain",
+                "content": {"membership": "join"},
+                "signatures": {},
+                "unsigned": {},
+            },
+        )
+
+        # After MSC3375, the join_authorised_via_users_server key is protected
+        # from redaction.
+        self.run_test(
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "join",
+                    "join_authorised_via_users_server": "@user:domain",
+                    "other_key": "stripped",
+                },
+            },
+            {
+                "type": "m.room.member",
+                "content": {
+                    "membership": "join",
+                    "join_authorised_via_users_server": "@user:domain",
+                },
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.V9,
+        )
+
 
 class SerializeEventTestCase(unittest.TestCase):
     def serialize(self, ev, fields):