A new species of triplefin, Enneapterygius viridicauda is described on the basis of two specimens from southwestern Flores, Indonesia, in the eastern Indian Ocean. The new species is characterized within the genus by 19 anal-fin soft rays, 7 median branched pectoral-fin rays, 15–16+19 lateral-line scales, a mandibular-pore formula of 4+1+4, eye diameter 100–119 (thousandths of SL), preorbital 69–72, body depth 211–225, preanal fin length 489–500. The preserved male has a black head-mask, a dark posterior half of the body, a pale third dorsal fin (only basally dusky), a plain black anal fin, no triangular black blotches above the anal-fin base, pale pelvic fins, and a dark caudal fin with a narrow pale margin. In life, male E. viridicauda have a distinctive green rear body and tail with the anterior body rose; a dark mask over the head, a black anal fin, and a row of dark spots along the anterior lateral line.