A new cottid species, Icelus sekii, is described on the basis of six specimens collected from off Rausu and Urakawa, Hokkaido Island, Japan. This species is distinguished from its congeners by the following combination of characters: supraocular and parietal spines absent; nuchal spine obscure; uppermost preopercular spine unbranched; no scales between dorsal scale row and lateral-line scale row, and no scales below lateral-line scale row; supraocular, parietal, and nuchal cirri present; five dark brown saddles dorsolaterally; anal-fin rays 13; pectoral-fin rays 15; vertebrae 12 + 24~5 = 36~7. Icelus sekii can be mature at the smallest size among the species of Icelus. As a secondary sexual character, the male holotype has unique ensiform flaps on the distal tips of the first dorsal fin.