Heteromysoides kumejimensis, a new species of the family Mysidae (Mysida) is described from a submarine cave on Kumejima Island, Ryukyu Islands, southwestern Japan, This species can be distinguished from other species of the genus by the following combination of characters: eyes subglobular in dorsal aspect, without sharp process; cornea developed; telson subtriangular, distally rounded, 1.7 times as long as maximum width, with 3 or 4 pairs of short simple spines laterally, and with 3 or 4 pairs of long serrate spines posterolaterally and 1 pair of short simple spines apically; and endopod of third thoracopod with simple, not subchelate, termination.