The tetrarogid waspfish Ocosia sphex n. sp. is described on the basis of a single specimen that was trawled from a steep rocky slope with gorgonian corals in 155–120 m depth at New Hanover, Bismarck Archipelago, Papua New Guinea. It is characterized by 14 spines and 8 soft rays in the dorsal fin, the last ray divided; 3 spines and 6 soft rays in the anal fin, the last ray divided; 13 pectoral-fin rays; 3+8=11 gill rakers (some rudimentary); 5 preopercular spines; 26–27 lateral-line scales; the second and third dorsal-fin spines not markedly elongate relative to succeeding spines; the membranes of the mid-spinous portion of the dorsal fin incised for one-fourth to one-third of length of the succeeding spine; the origin of the dorsal fin at or about the level of the middle of the eye; the first lachrymal spine about one-third the length of the second spine, pointing downward and out rather than back; and minute stubby papillae confined to the anteriormost part of premaxillary or absent. A key to the eight known species of Ocosia is presented.