The suborder Colomastigidea consists essentially of small commensal amphipods and, to the date, no species are known from Brazilian waters. Here, a new species (Colomastix trispinosa sp. nov.) of the genus Colomastix is described from the Northeastern Brazilian coast, Ceará state. The new species is distinguished from the others in the genus especially by the male morphology of the palm of gnathopod 2 and the apex of inner ramus of uropod 1, as well as for the combination of other characters. In addition, it is the first Atlantic species to have a pleated apex on dactylus of female gnathopod 2.