The Didessa is a river in western Ethiopia. A tributary of the Abay River, it rises in the mountains of Gomma, flowing in a northwesterly direction to its confluence where the course of the Abay has curved to its southernmost point before turning northwards. The Didessa's drainage area is about 25,800 km2 (10,000 sq mi), covering portions of the Benishangul-Gumuz Region and the West Welega Zone of the Oromia Region.