A juvenile of the slender fangjaw, Sigmops gracilis G$ddot{u}$nther, 1878 (Stomiiformes: Gonostomatidae) was collected from Jeju Island, Korea, and identified by DNA barcoding. This species is characterized by its large curved mouth and the presence of 11 dorsal fin rays and 28 anal fin rays. During the juvenile stage, the species is distinguished from other gonostomatid species by the position of the origin of the dorsal fin, which is located at the 7th-8th ray of the anal fin. The Korean name "Sol-ni-ael-tung-i" is proposed for this species.