AbstractA new species of marine fish leeches is described and compared with its congeners. Oceanobdella sakhalinica possesses a unique combination of characters: the anterior sucker is small; one of posterior transverse bands on the urosome is very dark; a wedge-shaped patch is located on the posterior sucker; posterior crop caeca are separate for their entire length; accessory glands on the atrium are lacking; five pairs of testisacs are present; the copulatory area and the conductive tissue are lacking. Hosts include a sculpin Myoxocephalus sp. and the blackline prickleback Acantholumpenus mackayi. The leeches were collected in Aniva Bay, southern Sakhalin.