A new species of the mysid crustacean genus Gastrosaccus Norman, 1868 (Mysida, Mysidae, Gastrosaccinae) is reported from a sandy shore of Lombok Island, Indonesia. These specimens resemble G. sorrentoensis Wooldridge & McLachlan, 1986 and G. yuyu Bamber and Morton, 2012 by the possession of an articulated process on the fifth abdominal somite together with a fringe of spine-like filaments on the posterodorsal margin of the carapace. The Lombok population differs from the known congeners by having comparatively fewer numbers of carpopropod segments on the endopod of the third to eighth thoracic limbs and the conformation in the telson and in the male third pleopod. Hence, G. lombokiensis sp. n. is proposed herein as a third species of "G. sorrentoensis" species group.