The Biwa trout (Oncorhynchus rhodurus) is an anadromous salmonid fish of the genus Oncorhynchus, endemic to Lake Biwa in Shiga Prefecture, Japan, but also introduced to Lake Ashi and Lake Chūzenji. While called trout, the fish is most closely related to the masu salmon Oncorhynchus masou of the western Pacific, and is most often considered a subspecies of it, Oncorhynchus masou rhodurus.