diff --git a/changelog.d/14369.bugfix b/changelog.d/14369.bugfix
new file mode 100644
index 000000000000..e6709f4eecda
--- /dev/null
+++ b/changelog.d/14369.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where Synapse would raise an error when encountering an unrecognised field in a `/sync` filter, instead of ignoring it for forward compatibility.
diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 26be377d0334..a9888381b451 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -43,7 +43,7 @@
     from synapse.server import HomeServer
 
 FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "limit": {"type": "number"},
@@ -63,7 +63,7 @@
 }
 
 ROOM_FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "not_rooms": {"$ref": "#/definitions/room_id_array"},
@@ -77,7 +77,7 @@
 }
 
 ROOM_EVENT_FILTER_SCHEMA = {
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
     "type": "object",
     "properties": {
         "limit": {"type": "number"},
@@ -143,7 +143,7 @@
             },
         },
     },
-    "additionalProperties": False,
+    "additionalProperties": True,  # Allow new fields for forward compatibility
 }