Acanthoplesiops naka sp. n. is described from the 9.9 mm SL holotype collected at Ofolanga Island of the Ha'apai Group of Tonga. Its dorsal-fin spine count of XVIII is unique in the genus. The fol-lowing combination of characters also distinguishes it from congeners: first two dorsal-fin pterygio-phores in the 3 rd interneural space (anterior dorsal-fin formula ?/?/I+I/I/I/I/I//I/I/I); 3 dentary pores; 2 intertemporal pores; head and body generally brownish with darker pigment spots (from shrunken melanophores), excepting a paler stripe from the anterior half of the symphyseal flap on the lower lip to the origin of the dorsal fin (continuing on to the first dorsal spine), a pale spot on the pectoral-fin base, and a pale caudal peduncle continuous with pale bases of last 3 segmented dorsal-and anal-fin rays; regular ctenoid scales; no membranous attachment of last dorsal-or anal-fin rays to caudal fin. The new species might be the sister taxon to A. psilogaster as they share the putative derived character of two dorsal-fin pterygiophores in the 3 rd interneural space.