The Cavalla River (also known as Cavally, Youbou or Diougou river) is a river in West Africa running from north of Mont Nimba in Guinea, through Côte d'Ivoire, to Zwedru in Liberia, and back to the border with Côte d'Ivoire. It ends in the Gulf of Guinea 21 km (13 mi) east of Harper, Liberia. It forms the southern two-thirds of the international boundary between Liberia and Côte d'Ivoire.
It has a length of 515 kilometres (320 mi), and is the longest river in Liberia. The name is derived from the cavalla horse mackerel found at its mouth.