diff --git a/changelog.d/6082.feature b/changelog.d/6082.feature
new file mode 100644
index 0000000000..c30662b608
--- /dev/null
+++ b/changelog.d/6082.feature
@@ -0,0 +1 @@
+Return 403 on `/register/available` if registration has been disabled.
\ No newline at end of file
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 34276ea3fa..e99b1f5c45 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -328,6 +328,11 @@ class UsernameAvailabilityRestServlet(RestServlet):
 
     @defer.inlineCallbacks
     def on_GET(self, request):
+        if not self.hs.config.enable_registration:
+            raise SynapseError(
+                403, "Registration has been disabled", errcode=Codes.FORBIDDEN
+            )
+
         ip = self.hs.get_ip_from_request(request)
         with self.ratelimiter.ratelimit(ip) as wait_deferred:
             yield wait_deferred