diff --git a/compose.yml b/compose.yml
index d349333..09f782e 100644
--- a/compose.yml
+++ b/compose.yml
@@ -128,6 +128,10 @@ services:
     secrets:
       - db_password
     environment:
+      # postgres:18 relocated the default PGDATA to /var/lib/postgresql/18/docker.
+      # Pin it to the legacy path so pgautoupgrade finds the existing cluster on the
+      # mounted volume and upgrades it in place, instead of init'ing an empty one.
+      - PGDATA=/var/lib/postgresql/data
       - LC_COLLATE=C
       - LC_CTYPE=C
       - POSTGRES_DB=synapse