The Garonne (/ɡəˈrɒn, ɡæˈrɒn/ gə-RON, garr-ON, French: [ɡaʁɔn] ^ⓘ; Catalan, Basque and Occitan: Garona, Occitan pronunciation: [ɡaˈɾunɔ]; Latin: Garumna or Garunna) is a river that flows in southwest France and northern Spain. It flows from the central Spanish Pyrenees to the Gironde estuary at the French port of Bordeaux – a length of 529 km (329 mi), of which 47 km (29 mi) is in Spain (Val d'Aran); the total length extends to 602 km (374 mi) if one includes the Gironde estuary between the river and the sea. Its basin area is 56,000 km² (22,000 sq mi), which increases to 84,811 km² (32,746 sq mi) if the Dordogne River, which flows from the east and joins the Garonne at Bec d'Ambès to form the Gironde estuary, is included.

Umayyad and Aquitanian forces fought the Battle of the River Garonne in 732 beside the river near present-day Bordeaux.

The name derives from Garumna, a Latinized version of the Aquitanian name meaning "stony river" ("kar" relating to "stone" and "-ona" relating to "river").

The Garonne's headwaters are to be found in the Aran Valley in the Spanish Pyrenees, though three different locations have been proposed as the true source: the Uelh deth Garona at Plan de Beret (42°42′34″N 0°56′43″E / 42.709494°N 0.945398°E / 42.709494; 0.945398), the Ratera-Saboredo cirque 42°36′26″N 0°57′56″E / 42.607295°N 0.965424°E / 42.607295; 0.965424), or the slopes of Pic Aneto (Salterillo-Barrancs ravine 42°38′59″N 0°40′06″E / 42.6498°N 0.6683°E / 42.6498; 0.6683 according to the season).

The Uelh deth Garona at 1,862 metres (6,109 ft) above sea level has been traditionally considered as the source of the Garonne. From this point a brook (called the Beret-Garona) runs for 2.5 kilometres (1.6 mi) to the bed of the main upper Garonne valley. The river runs for another 38 kilometres (24 mi) to the French border at Pont de Rei, 40.5 kilometres (25.2 mi) in total.

The Ratera-Saboredo cirque is the head of the upper Garonne valley, and its upper lake at 2,600 metres (8,500 ft) above sea level is the origin of the Ruda-Garona river, running for 16 kilometres (9.9 mi) to the confluence with the Beret-Garona brook and another 38 kilometres (24 mi) to the French border at Pont del Rei, 54 kilometres (34 mi) in total. At the confluence, the Ruda-Garona carries 2.6 cubic metres per second (92 cu ft/s) of water. The Ratera-Saboredo cirque is considered by many researchers to be the origin of the Garonne.

The third theory holds that the river rises on the slopes of Pic Aneto at 2,300 metres (7,500 ft) above sea level and flows by way of a sinkhole known as the Forau de Aigualluts (42°40′00″N 0°40′01″E / 42.6666°N 0.6669°E / 42.6666; 0.6669) through the limestone of the Tuca Blanca de Pomèro and a resurgence in the Val dera Artiga above the Aran Valley in the Spanish Pyrenees. This underground route was suggested by the geologist Ramond de Carbonnières in 1787 but there was no confirmation until 1931, when caver Norbert Casteret poured fluorescein dye into the flow and noted its emergence a few hours later 4 kilometres (2.5 mi) away at Uelhs deth Joèu ("Jove's eyes" 42°40′51″N 0°42′28″E / 42.68092°N 0.7077°E / 42.68092; 0.7077) in the Artiga de Lin on the other side of the mountain. From Aigualluts to the confluence with the main river at the bed of the upper Garonne valley at 800 metres (2,600 ft) above sea level, the Joèu has run for 12.4 kilometres (7.7 mi) (16 kilometres more to get to the French border), carrying 2.16 cubic metres per second (76 cu ft/s) of water, whilst the main river is carrying 17.7 cubic metres per second (630 cu ft/s).

Despite the lack of universal agreement on definition for determining a stream's source, the United States Geological Survey, the National Geographic Society and the Smithsonian Institution agree that a stream's source should be considered as the most distant point (along watercourses from the river mouth) in the drainage basin from which water runs.