A new kalliapseudid species, Mesokalliapseudes quadriflagellata n. sp. from Puerto Rico is described and illustrated. It can be distinguished from its congeners by the following combination of characteristics: 1) rounded rostrum, 2) pereonites lacking anterolateral apophyses, 3) fixed finger of cheliped two-thirds or more the length of the dactylus, and 4) serrate spiniform seta on the basis of pereopod 1. It is unique in having the inner flagellum of the antennule with 4 articles (all other congeners have 3 articles). Remarks on the genus Mesokalliapseudes are presented.