The Sava is a river in Southeast Europe, a right side tributary of the Danube river at Belgrade. Counting from Zelenci, the source of Sava Dolinka, it is 945 kilometres (587 mi) long - the second biggest catchment of Danube tributaries after Tisza and drains 97,713 square kilometres (37,727 sq mi) of surface area. It flows through Slovenia, Croatia, along the northern border of Bosnia and Herzegovina, and through Serbia. Its middle basin is a natural border of Bosnia and Herzegovina and Croatia. Sava is considered to be the northern border of the Balkan Peninsula.
It belongs to the Black Sea drainage basin, and represents the longest Danube's right tributary and the second longest of all, as well as the richest with water, by far. It drains a significant portion of the Dinaric Alps region, through significant tributaries of Kupa, Una, Vrbas, Bosna and Drina. Sava also the 16th longest river in Europe and the second longest one (after Tisza) that does not join directly a sea.