Accacoelium contortum is a parasitic flatworm of the class Trematoda. It lives in the gills and oral cavity of the ocean sunfish, Mola mola, and was first described by Swedish-born naturalist Karl Rudolphi in 1819. Accacoelium contortum is one of the most common sunfish parasites and has been reported in the Mediterranean Sea, the Northeast Atlantic Ocean and the South Pacific. In a study in Spain, 47.2% of 106 sunfish examined were found to host Accacoelium contortum.