The Aheloy (Bulgarian: Ахелой), also known as the Achelous, is a river in eastern Bulgaria. It is 40 km long. The river is famous for being the site of the Battle of Achelous that took place on 20 August 917 between Bulgarian ruler Simeon I and the Byzantines under Leo Phocas during the Byzantine–Bulgarian war of 913–927. It was one of the largest battles in the Middle Ages and among the greatest military successes of the First Bulgarian Empire.
The main stem is the Arnautska reka, which springs from the Aytoska Planina division of the eastern Balkan Mountains east of the village of Dryankovets. The river proper is formed from the confluence of the Arnautska reka with the Mangarska reka at an altitude of 166 m, about three kilometers east of the village of Belodol. The Aheloy flows eastwards in a wide alluvial valley until the village of Aleksandrovo, where it turns southeast for the remainder of its length. It flows into the Gulf of Burgas of the Black Sea near a campsite some 1.2 km south of the town of Aheloy.