diff --git a/nixos/modules/programs/xss-lock.nix b/nixos/modules/programs/xss-lock.nix
index a7ad9b89db4d..83ed71386407 100644
--- a/nixos/modules/programs/xss-lock.nix
+++ b/nixos/modules/programs/xss-lock.nix
@@ -34,7 +34,7 @@ in
       partOf = [ "graphical-session.target" ];
       serviceConfig.ExecStart = with lib;
         strings.concatStringsSep " " ([
-            "${pkgs.xss-lock}/bin/xss-lock"
+            "${pkgs.xss-lock}/bin/xss-lock" "--session \${XDG_SESSION_ID}"
           ] ++ (map escapeShellArg cfg.extraOptions) ++ [
             "--"
             cfg.lockerCommand
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index b8b36aa05324..08ce8edd6612 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -55,13 +55,6 @@ let
         exec &> >(tee ~/.xsession-errors)
       ''}
 
-      # Tell systemd about our $DISPLAY and $XAUTHORITY.
-      # This is needed by the ssh-agent unit.
-      #
-      # Also tell systemd about the dbus session bus address.
-      # This is required by user units using the session bus.
-      /run/current-system/systemd/bin/systemctl --user import-environment DISPLAY XAUTHORITY DBUS_SESSION_BUS_ADDRESS
-
       # Load X defaults. This should probably be safe on wayland too.
       ${xorg.xrdb}/bin/xrdb -merge ${xresourcesXft}
       if test -e ~/.Xresources; then
@@ -70,6 +63,12 @@ let
           ${xorg.xrdb}/bin/xrdb -merge ~/.Xdefaults
       fi
 
+      # Import environment variables into the systemd user environment.
+      ${optionalString (cfg.displayManager.importedVariables != []) (
+        "/run/current-system/systemd/bin/systemctl --user import-environment "
+          + toString (unique cfg.displayManager.importedVariables)
+      )}
+
       # Speed up application start by 50-150ms according to
       # http://kdemonkey.blogspot.nl/2008/04/magic-trick.html
       rm -rf "$HOME/.compose-cache"
@@ -289,6 +288,14 @@ in
         '';
       };
 
+      importedVariables = mkOption {
+        type = types.listOf (types.strMatching "[a-zA-Z_][a-zA-Z0-9_]*");
+        visible = false;
+        description = ''
+          Environment variables to import into the systemd user environment.
+        '';
+      };
+
       job = {
 
         preStart = mkOption {
@@ -393,6 +400,16 @@ in
 
     services.xserver.displayManager.xserverBin = "${xorg.xorgserver.out}/bin/X";
 
+    services.xserver.displayManager.importedVariables = [
+      # This is required by user units using the session bus.
+      "DBUS_SESSION_BUS_ADDRESS"
+      # These are needed by the ssh-agent unit.
+      "DISPLAY"
+      "XAUTHORITY"
+      # This is required to specify session within user units (e.g. loginctl lock-session).
+      "XDG_SESSION_ID"
+    ];
+
     systemd.user.targets.graphical-session = {
       unitConfig = {
         RefuseManualStart = false;