diff --git a/changelog.d/18875.bugfix b/changelog.d/18875.bugfix
new file mode 100644
index 000000000..3bda7a1d1
--- /dev/null
+++ b/changelog.d/18875.bugfix
@@ -0,0 +1 @@
+Fix prefixed support for MSC4133.
diff --git a/synapse/rest/client/profile.py b/synapse/rest/client/profile.py
index 243245f73..8bc532c81 100644
--- a/synapse/rest/client/profile.py
+++ b/synapse/rest/client/profile.py
@@ -109,6 +109,12 @@ class ProfileFieldRestServlet(RestServlet):
         self.hs = hs
         self.profile_handler = hs.get_profile_handler()
         self.auth = hs.get_auth()
+        if hs.config.experimental.msc4133_enabled:
+            self.PATTERNS.append(
+                re.compile(
+                    r"^/_matrix/client/unstable/uk\.tcpip\.msc4133/profile/(?P<user_id>[^/]*)/(?P<field_name>[^/]*)"
+                )
+            )
 
     async def on_GET(
         self, request: SynapseRequest, user_id: str, field_name: str
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index 7f7837953..1b8efd98c 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -175,6 +175,7 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.simplified_msc3575": msc3575_enabled,
                     # Arbitrary key-value profile fields.
                     "uk.tcpip.msc4133": self.config.experimental.msc4133_enabled,
+                    "uk.tcpip.msc4133.stable": True,
                     # MSC4155: Invite filtering
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
                     # MSC4306: Support for thread subscriptions