The crustacean order Thermosbaenacea is reported for the first time from the Southern Hemisphere, from almost fresh water in a cave habitat in tropical Western Australia. Halosbaena tulki, sp. nov. belongs to a genus previously known only from saline waters in the West Indies, Columbia and Canary Is. The discovery is consistent with a very ancient origin of the order and distribution of the genus by plate movements following the breakup of Pangaea.