diff --git a/actionpack/lib/action_dispatch/middleware/ssl.rb b/actionpack/lib/action_dispatch/middleware/ssl.rb
index 735b5939dd44a85a47a2cf9370674b5e4c046149..823cefe9576e98eb5eb21c36c672b0291a0d8e51 100644
--- a/actionpack/lib/action_dispatch/middleware/ssl.rb
+++ b/actionpack/lib/action_dispatch/middleware/ssl.rb
@@ -40,7 +40,7 @@ class SSL
     HSTS_EXPIRES_IN = 15552000
 
     def self.default_hsts_options
-      { expires: HSTS_EXPIRES_IN, subdomains: false, preload: false }
+      { expires: HSTS_EXPIRES_IN, subdomains: true, preload: false }
     end
 
     def initialize(app, redirect: {}, hsts: {}, secure_cookies: true, **options)