diff --git a/modules/services/synapse.nix b/modules/services/synapse.nix
index fa16979b..a107586a 100644
--- a/modules/services/synapse.nix
+++ b/modules/services/synapse.nix
@@ -112,6 +112,7 @@ inputs:
               in builtins.readFile ((inputs.pkgs.formats.yaml {}).generate "${instance.name}.yaml"
               {
                 server_name = instance.value.matrixHostname;
+                public_baseurl = "https://${instance.value.hostname}";
                 listeners =
                 [{
                   bind_addresses = [ "127.0.0.1" ];