diff --git a/roles/custom/matrix-synapse/defaults/main.yml b/roles/custom/matrix-synapse/defaults/main.yml
index 0878877ce..2a62bf5b8 100644
--- a/roles/custom/matrix-synapse/defaults/main.yml
+++ b/roles/custom/matrix-synapse/defaults/main.yml
@@ -1423,6 +1423,16 @@ matrix_synapse_max_event_delay_duration: 24h
 # See https://github.com/matrix-org/matrix-spec-proposals/pull/4222
 matrix_synapse_experimental_features_msc4222_enabled: false
 
+# Controls whether to enable the MSC4306 experimental feature ("thread subscriptions").
+#
+# In current Synapse, this also enables the MSC4308 thread-subscriptions extension
+# to Sliding Sync under the same upstream feature flag.
+#
+# See:
+# - https://github.com/matrix-org/matrix-spec-proposals/pull/4306
+# - https://github.com/matrix-org/matrix-spec-proposals/pull/4308
+matrix_synapse_experimental_features_msc4306_enabled: false
+
 # Controls whether to enable the MSC4354 experimental feature (sticky events).
 #
 # This is implemented since Synapse v1.148.0 and can be used by element-call v0.17.0+
diff --git a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2 b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
index c29948bd2..055dc9ec8 100644
--- a/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
+++ b/roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2
@@ -3006,6 +3006,9 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4222_enabled %}
   msc4222_enabled: true
   {% endif %}
+  {% if matrix_synapse_experimental_features_msc4306_enabled %}
+  msc4306_enabled: true
+  {% endif %}
   {% if matrix_synapse_experimental_features_msc4354_enabled %}
   msc4354_enabled: true
   {% endif %}