We describe the shell, soft anatomy, and reproduction of the montacutid bivalve, Koreamya arcuata (A. Adams, 1856) gen. nov. Contrary to most montacutids the outer demibranch has been preserved, although in a reduced state. The species is a protandrous consecutive hermaphrodite which stores sperm in a groove-shaped ctenidial seminal receptacle. The particular type of sperm receptacle and the presence of a lithodesma in Koreamya indicate an affinity to Montacutona Yamamoto & Habe, 1959, but the shells of the two differ widely in shape and proportions. The species is a commensal, and lives permanently attached to the valves near the shell gape of Lingula anatina at several Korean intertidal flats. It is also known from the Philippines, SE India and W Australia.