The Sava (; Slovene pronunciation: [ˈsàːʋa], Serbo-Croatian pronunciation: [sǎːʋa]; Serbian Cyrillic: Сава, Hungarian: Száva) is a river in Central and Southeast Europe, a right-bank and the longest tributary of the Danube. It flows through Slovenia, Croatia and along its border with Bosnia and Herzegovina, and finally through Serbia, feeding into the Danube in its capital, Belgrade. The Sava forms the main northern limit of the Balkan Peninsula, and the southern edge of the Pannonian Plain.

The Sava is 990 kilometres (615 miles) long, including the 45-kilometre (28 mi) Sava Dolinka headwater rising in Zelenci, Slovenia. It is the largest tributary of the Danube by volume of water, and second-largest after the Tisza in terms of catchment area (97,713 square kilometres (37,727 square miles)) and length. It drains a significant portion of the Dinaric Alps region, through the major tributaries of Drina, Bosna, Kupa, Una, Vrbas, Lonja, Kolubara, Bosut and Krka. The Sava is one of the longest rivers in Europe and among the longest tributaries of another river.

The population in the Sava River basin is estimated at 8,176,000, and is shared by three capital cities: Ljubljana, Zagreb and Belgrade. The Sava is about 2⁄3-navigable for larger vessels: from the confluence of the Kupa in Sisak a few kilometers below Zagreb.

The name is believed to be derived from the Proto-Indo-European root *sewh₁ ('to take liquid', whence the English word sup) and the ending *eh₂, so that it literally means 'that which waters [the ground]'. The ancient Greeks called it Saos (Ancient Greek: Σάος).