A new species of small-sized scorpionfish, Scorpaena cocosensis, is described on the basis of a single specimen collected from off Nuez Island, Cocos Islands, Costa Rica, eastern Pacific Ocean. The new species is similar to two eastern Pacific species of Scorpaena, Scorpaena russula and Scorpaena sonorae, in overall body appearance and in lacking a supplemental preopercular spine. However, it is distinguished from these two species by the following characters: eight dorsal-fin soft rays; well-exposed scales covering anteroventral body surface; interorbital ridges well developed, beginning just behind nasal spines, diverging anteriorly and posteriorly in dorsal view; lateral margins of frontal diverging posteriorly in dorsal view; upper posttemporal spine directed upward; posterior margin of maxilla just reaching level with posterior margin of pupil; posterior tip of pectoral fin reaching level with origin of third dorsal-fin soft ray; large head length (48.8% SL); large orbit diameter (16.9% SL); wide interorbital space between supraocular spine bases (9.2% SL).