diff --git a/modules/system/defaults/NSGlobalDomain.nix b/modules/system/defaults/NSGlobalDomain.nix
index 7e6119427da9f..f5c0a5c8d9b9b 100644
--- a/modules/system/defaults/NSGlobalDomain.nix
+++ b/modules/system/defaults/NSGlobalDomain.nix
@@ -69,6 +69,14 @@ with lib;
       '';
     };
 
+    system.defaults.NSGlobalDomain.NSAutomaticPeriodSubstitutionEnabled = mkOption {
+      type = types.nullOr types.bool;
+      default = null;
+      description = ''
+        Whether to enable smart period substitution.  The default is true.
+      '';
+    };
+
     system.defaults.NSGlobalDomain.NSAutomaticSpellingCorrectionEnabled = mkOption {
       type = types.nullOr types.bool;
       default = null;