A new species, Arthula biformis Oh and Lee sp. nov., belonging to the cryptine ichneumonid subtribe Sphecophagina Beirne is described from Korea. The new species is composed of two seasonal forms, overwintering and summer, distinguished by color and some morphological features. The overwintering form is much darker than the summer form. Keys to the genera of Sphecophagina and the known species of Arthula Cameron and photographs of diagnostic characters of both color forms of the new species are provided.