A new species of the genus Nihonotrypaea Manning & Tamaki, 1998, Nihonotrypaea hainanensis sp. n., collected from the South China Sea, is described and illustrated. It is distinguishable from Nihonotrypaea harmandi (Bouvier, 1901), Nihonotrypaea japonica (Ortmann, 1891), Nihonotrypaea thermophila Lin, Komai & Chan, 2007 and Nihonotrypaea makarovi Martin, 2013 by having the elongated carpus of the male and female major cheliped. The new species is distinguishable from Nihonotrypaea petalura (Stimpson, 1860) by the proximolower margin of the carpus of the male major cheliped bearing several small denticles.