Aiming at a better understanding of the harpacticoid diversity of the South China Sea, sediment samples were taken from Nha Trang Bay (Vietnam) during a sampling campaign carried out in April, 2004. Two female specimens of the monotypic genus Sentiropsis were found. These turned out to belong to a new species, Sentiropsis vietnamensis n. sp., which can be separated from the type species S. minuta by the shape of the setae of the female antennule, size of the seta of the second segment of the antennary exopod, lateral setation of the endopod of the mandible, armature of the arthrite of the maxillule, shape of the seta of the syncoxa of the maxilliped, shape of the seta of the basis of the maxilliped, ventral spinular ornamentation on the posterior half of the genital double-somite and fourth urosomite of the female, P1ENP1∶P1ENP2 length ratio, armature formula of the female P5EXP, shape of the apical setae EXP3 and ENP2, length ratio of the outer spines of P4EXP1 and EXP2, the shape and size of the endopodal lobe of P5, and length of the apical seta on the P5 endopodal lobe. The diagnosis of the genus is amended.