The Río Rosario is a river in the province of Salta, in northwestern Argentina, forming part of the broader Cuenca del Plata drainage system that ultimately reaches the Atlantic Ocean. The river's watershed covers approximately 4,780 km² and coincides closely with the Departamento Rosario de Lerma, situated in the center-north of the Valle de Lerma — the most densely populated region of Salta province.

The river originates at the Nevado de Chañi at an elevation of 5,896 meters above sea level and descends to approximately 1,170 meters at its mouth near El Carril, giving it a total course of around 210 km. In its upper reaches, the river is known as the Río Toro and flows through the Quebrada del Toro, a section characterized by steep gradients, a rocky channel, and mountainous terrain partially covered in high-altitude steppe vegetation. This upper corridor passes through an area of significant pre-Columbian heritage — the ancient city of Tastil, estimated to have housed around 2,000 inhabitants, is considered the largest pre-Columbian urban settlement in what is now Argentine territory.

Where the gradient eases at the end of the Quebrada del Toro, the river takes on the name Río Rosario. It then passes south of the town of Campo Quijano as it opens into the Valle de Lerma. Major tributaries include the Quebrada de Tastil, the Río Agua Chuya, and the Río Pulares. The mean discharge is 11.7 m³/s, though flows vary considerably by season. The regional climate is monsoonal, meaning rainfall is concentrated almost entirely in the warmer months, producing significantly higher water volumes in summer compared to winter. Paddlers should plan accordingly, as seasonal flow differences can substantially alter the character of the river.