diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index a8c9305704..416f6b240e 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -563,6 +563,9 @@ class ExperimentalConfig(Config):
         # MSC4133: Custom profile fields
         self.msc4133_enabled: bool = experimental.get("msc4133_enabled", False)
 
+        # MSC4429: Profile updates for legacy /sync
+        self.msc4429_enabled: bool = experimental.get("msc4429_enabled", False)
+
         # MSC4143: Matrix RTC Transport using Livekit Backend
         self.msc4143_enabled: bool = experimental.get("msc4143_enabled", False)
 
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index f8d7a1a4d9..3aae506eb4 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -198,6 +198,8 @@ class VersionsRestServlet(RestServlet):
                     # Arbitrary key-value profile fields.
                     "uk.tcpip.msc4133": self.config.experimental.msc4133_enabled,
                     "uk.tcpip.msc4133.stable": True,
+                    # MSC4429: Profile updates for legacy /sync.
+                    "org.matrix.msc4429": self.config.experimental.msc4429_enabled,
                     # MSC4155: Invite filtering
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
                     # MSC4306: Support for thread subscriptions