diff --git a/roles/custom/matrix-synapse/defaults/main.yml b/roles/custom/matrix-synapse/defaults/main.yml
index cd9cfa7ea58..7742f47643f 100644
--- a/roles/custom/matrix-synapse/defaults/main.yml
+++ b/roles/custom/matrix-synapse/defaults/main.yml
@@ -1252,6 +1252,15 @@ matrix_synapse_experimental_features_msc4108_enabled: false
 ################################################################################
 
 
+# Controls whether to enable the "room summary API" experimental feature.
+# See https://github.com/matrix-org/matrix-spec-proposals/pull/3266
+# Despite being experimental, this feature is mandatory for the next-generation Element X clients, which is why it is enabled by default:
+# https://github.com/element-hq/element-x-ios/issues/3713#issuecomment-2620958291
+# If you're worried about the privacy implications of this unauthenticated API, see:
+# https://github.com/deepbluev7/matrix-doc/blob/room-summaries/proposals/3266-room-summary.md#client-server-api
+# Set this to false if you still want to disable to API for some reason. Note that doing so breaks Element X compatibility though.
+matrix_synapse_experimental_features_msc3266_enabled: true
+
 # Enable this to activate the REST auth password provider module.
 # See: https://github.com/ma1uta/matrix-synapse-rest-password-provider
 matrix_synapse_ext_password_provider_rest_auth_enabled: false
diff --git a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2 b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
index aecaf03017b..dbfbdffaffa 100644
--- a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
+++ b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
@@ -2984,5 +2984,8 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4108_enabled %}
   msc4108_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc3266_enabled %}
+  msc3266_enabled: true
+  {% endif %}
 
 # vim:ft=yaml