The Soča or Isonzo is a 138-kilometre long river that flows through western Slovenia and northeastern Italy.
An Alpine river in character, its source lies in the Trenta Valley in the Julian Alps in northwestern Slovenia, at an elevation of 876 metres. The river runs past the towns of Bovec, Kobarid, Tolmin, Kanal ob Soči, Nova Gorica, and Gorizia, entering the Adriatic Sea close to the town of Monfalcone. It has a nival-pluvial regime in its upper course and pluvial-nival in its lower course.
Prior to the First World War, the river ran parallel to the border between Kingdom of Italy and the Austro-Hungarian Empire. During World War I, it was the scene of bitter fighting between the two countries, culminating in the Battle of Caporetto in 1917.