A taxonomic study of the subgenus Agathidium of the genus Agathidium Panzer in Korea is presented. Four species [Agathidium (Agathidium) coreanum Angelini, A. (A.) crassicorne Portevin, A. (A.) merkli Angelini, and A. (A.) oblongum Angelini] are recognized. Agathidium (A.) mequignoni Roubal recorded by Angelini is misidentification of A. (A.) crassicorne Portevin. Spermatheca of A. (A.) merkli is illustrated for the first time. A key to the species, diagnoses and illustrations of diagnostic characters are provided.