The Drina (Serbian Cyrillic: Дрина, pronounced [drǐːna]) is a 346 km (215 mi) long river in the Balkans, which forms a large portion of the border between Bosnia and Herzegovina and Serbia. It is the longest tributary of the Sava River and the longest karst river in the Dinaric Alps which belongs to the Danube River drainage basin.

The Drina originates from the confluence of the rivers Tara and Piva, in the glen between the slopes of the Maglić, Hum and Pivska Planina mountains, between the villages of Šćepan Polje, Montenegro and Hum, Bosnia and Herzegovina.

Its name is derived from the Roman name of the river (Latin: Drinus) which in turn is derived from Greek (Ancient Greek: Dreinos) which is derived from the native name of Illyrian origin.

The Drina is a very fast and cold alpine river, with a very high 175:346 meandering ratio, and relatively clean water, which has particularly intensive green coloration, a usual characteristic of most alpine rivers running through a karstic and flysch terrain made of limestone, underlying the area in which the river carved its bed.

Its average depth is 3 to 5 m (9.8 to 16.4 ft), the deepest being 12 m (39 ft) at Tijesno. On average, the Drina is 50–60 m (160–200 ft) wide, but it ranges from only 12–20 m (39–66 ft) at Tijesno to up to 200 m (660 ft) at Bajina Bašta and Ljubovija. The drainage basin covers 19,570 square km (4.8 million acres), branching into Bosnia and Herzegovina, Serbia, Montenegro, and Albania. The Drina belongs to the Black Sea drainage basin. Before it was regulated by several power stations, the Drina used to flood its valley. The most disastrous flood occurred in 1896, which destroyed the town of Ljubovija.

The Drina originates at the point of confluence of the rivers Tara and Piva, between the slopes of the Maglić, Hum and Pivska Planina mountains, and the villages of Šćepan Polje (in Montenegro) and Hum (Bosnia and Herzegovina). At its origin, it flows west, then makes a long curve to the northeast, around the Maluša Mountain. From here it is northbound, in terms of general direction, for the rest of its journey toward the Sava. Here, in its headwaters, the Drina receives the Sutjeska River from the left.

The Drina is formed by the confluence of the Tara and the Piva rivers, both of which flow from Montenegro and converge on the border of Bosnia and Herzegovina, at Hum and Šćepan Polje villages. The total length of the Tara river is 144 km (89 mi), of which 104 km (65 mi) are in Montenegro, while the final 40 km (25 mi) are in Bosnia and Herzegovina along which form the border between the two countries in several places. The Drina flows through Bosnia and Herzegovina northward for 346 km (215 mi), of which 206 km (128 mi) is along the border of Bosnia and Herzegovina and Serbia, and finally spills out into the Sava river near Bosanska Rača village in northeastern Bosnia and Herzegovina. Measured from the source of the Tara, its longer headwater, the Drina is 487 kilometers (303 miles) long.

Major left tributaries: Sutjeska (at Kosman), Bjelava (at Trbušće), Bistrica (at Brod na Drini), Kolunska rijeka (at Ustikolina), Osanica (at Osanica), Prača (at Ustiprača), Žepa (Žepa), Drinjača (at Drinjača), Kamenica (at Đevanje), Sapna (at Karakaj) and Janja (at Janja).