The Cavalla River (also known as the Cavally, the Youbou and the Diougou) is a river in West Africa, originating from north of Mont Nimba in Guinea, through Ivory Coast, to Zwedru in Liberia, and back to the border with Ivory Coast. It ends in the Gulf of Guinea, situated 21 km east of Harper, Liberia. It forms the southern two-thirds of the international boundary between Liberia and Ivory Coast. It has a length of 515 km, and is the longest river in Liberia. The name is derived from the cavalla horse mackerel found at its mouth. It is home to the endemic Chiloglanis normani.