diff --git a/kde-modules/KDEInstallDirs.cmake b/kde-modules/KDEInstallDirs.cmake
--- a/kde-modules/KDEInstallDirs.cmake
+++ b/kde-modules/KDEInstallDirs.cmake
@@ -133,11 +133,11 @@
 # ``JARDIR``
 #     Java AAR/JAR files for Android. Since 5.62.0
 # ``SYSTEMDUNITDIR``
-#     Systemd Units (``LIBDIR/systemd``)
-#     [``SYSTEMD_UNIT_INSTALL_DIR``]. Since 5.FIXME
+#     Systemd Units (``lib/systemd``)
+#     [``SYSTEMD_UNIT_INSTALL_DIR``]. Since 5.65
 # ``SYSTEMDUSERUNITDIR``
-#     Systemd User Units (``LIBDIR/systemd/user``)
-#     [``SYSTEMD_USER_UNIT_INSTALL_DIR``]. Since 5.FIXME
+#     Systemd User Units (``lib/systemd/user``)
+#     [``SYSTEMD_USER_UNIT_INSTALL_DIR``]. Since 5.65
 #
 # If ``KDE_INSTALL_USE_QT_SYS_PATHS`` is set to TRUE before including this
 # module, the default values for some variables are instead queried from
@@ -242,7 +242,7 @@
 # GNUInstallDirs code deals with re-configuring, but that is dealt with
 # by the _define_* macros in this module).
 set(_LIBDIR_DEFAULT "lib")
-# Override this default 'lib' with 'lib64' iff:
+# Override this default 'lib' with 'lib64' if:
 #  - we are on a Linux, kFreeBSD or Hurd system but NOT cross-compiling
 #  - we are NOT on debian
 #  - we are NOT on flatpak
@@ -646,7 +646,7 @@
 _define_relative(DBUSSYSTEMSERVICEDIR DBUSDIR "system-services"
     "D-Bus system services"
     DBUS_SYSTEM_SERVICES_INSTALL_DIR)
-_define_relative(SYSTEMDUNITDIR LIBDIR "systemd"
+_define_relative(SYSTEMDUNITDIR CMAKE_INSTALL_PREFIX "lib/systemd"
     "Systemd units"
     SYSTEMD_UNIT_INSTALL_DIR)
 _define_relative(SYSTEMDUSERUNITDIR SYSTEMDUNITDIR "user"