A new species of triplefin (Tripterygiidae), Enneapterygius olivaceus n. sp., is described on the basis of 28 type specimens (13.7–25.1 mm standard length: SL) from Japan (Ryukyu Islands) and the Philippines. The new species is characterized by having 18–22 (20) notched lateral-line scales, 2 or 3 (2) scales below the 1st notched lateral-line scale, a single symphyseal mandibular-pore (mandibular pore formula 3 + 1 + 3), black pectoral fins in nuptial males, and no distinct line on the anal-fin membranes. In this study, Enneapterygius minutus (Günther, 1877), previously regarded as a junior synonym of Enneapterygius philippinus (Peters, 1868), is considered a valid species, being redescribed based on 109 specimens, including the re-examined type specimens. Diagnostic characters and distributional records of the species closely related to E. olivaceus n. sp. are reviewed. Enneapterygius tusitalae Jordan & Seale, 1906, Enneapterygius pardochir Jordan & Seale, 1906 and Enneapterygius punctulatus Herre, 1935 are regarded here as junior synonyms of E. minutus; Tripterygium callionymi Weber, 1909 and Enneapterygius waigiensis Herre, 1935 are junior synonyms of Enneapterygius tutuilae Jordan & Seale, 1906; Enneapterygius cerasinus Jordan & Seale, 1906 is a junior synonym of Enneapterygius unimaculatus Fricke, 1994; Enneapterygius fuligicauda Fowler, 1946 is a senior synonym of Enneapterygius similis Fricke, 1997.