A new species of lionfish (Scorpaenidae: Pteroinae), Pterois paucispinula sp. nov., is described on the basis of 37 specimens from the western Pacific Ocean. The new species is closely related to and has been previously confused with Pterois mombasae (Smith 1957), both species sharing usually XIII, 10 dorsal-fin rays, usually more than 18 pectoral-fin rays, numerous black blotches on the pectoral-fin membrane, and several bands on the posterior portion of the pectoral-fin rays (free from membrane). However, P. paucispinula differs from similarly sized P. mombasae in having a lesser body depth at the anal-fin origin, head width, postorbital length, and caudal-peduncle depth, in addition to a slightly higher number of scale rows below the lateral line. Pterois paucispinula also differs from the typical form of P. mombasae, occurring off the east coast of Africa and in the central Indian Ocean and Andaman Sea, by having usually 18 pectoral-fin rays (vs. usually 19 in the latter) and relatively long dorsal-fin spines, with the longest dorsal-fin spine length 42.9–51.7 % of standard length (SL) (vs. 35.1–44.8 % of SL). Although a Sri Lankan population of P. mombasae is similar to P. paucispinula in the above characters (usually 18 pectoral-fin rays and longest dorsal-fin spine length 37.5–51.1 % of SL), such differences among P. mombasae are regarded as intra-specific geographical variations. Additionally, young and adult P. mombasae have ctenoid scales on the pectoral-fin base, ventrolateral portion of the body (below the lateral line) and laterally on the caudal peduncle. These regions in all examined P. paucispinula usually have only cycloid or at most a few ctenoid scales, thereby providing a consistent basis for identification of both species, including the Sri Lankan population of P. mombasae. Pterois mombasae is distributed in the Indian Ocean from the east coast of Africa to the Andaman Sea, whereas P. paucispinula is recorded from the western Pacific Ocean, from northern Australia to southern Japan and eastward to the Wallis and Futuna Islands.
