Grammonus nagaredai is described as a new species of viviparous by-thitid fish from two specimens collected from caves off O‘ahu and Hawai‘i at depths of 6–9 m. Grammonus yunokawai Nielsen, known from one specimen from a cave in 20 m in the Ryukyu Islands, is the most similar species, differing in having a deeper body, more convex nape, broader maxilla, longer predorsal length, and shorter pectoral fins.