Five taxa of the genus Ajuga L. (Lamiaceae) were known previously in Korea which were mainly distributed in lowland and lower montane areas at 50-1000 m elevation. We report an unrecorded taxon of the genus Ajuga in Korea, namely Ajuga nipponensis Makino. This taxon was discovered at Is. Nok, Boryeong-si, in Chungcheongnam Province. A. nipponensis is found to be distributed along roadsides near villages of the island. A. nipponensis was distinguishable from other Ajuga in Korea by following combination of characters: Stems erect or procumbent, arranged in a dense cluster, corolla whitish-pale pink, 10-12 mm long. This taxon was named 'Bun-hong-kkot-jo-gae-na-mul' in Korean based on color of the corolla. The key to the genus Ajuga in Korea is also provided.