| San Juan River | |
| Map: | Rio San Juan.png |
| Source1 Location: | Confluence of the Calingasta, Castaño Viejo, and los Patos rivers |
| Mouth Location: | Desaguadero River |
| Progression: | Desaguadero River Atlantic Ocean |
| Subdivision Type1: | Country |
| Subdivision Name1: | Argentina |
| Subdivision Type2: | Province |
| Subdivision Name2: | San Juan |
| Length: | 500km (300miles) |
| Discharge1 Avg: | 56m3/s |
| Basin Size: | 39906km2 |
The San Juan River is, together with the Jáchal, the most important river of the Argentine province of San Juan. Both join the Desaguadero/Colorado system that ends at the Atlantic Ocean.
The river starts near the town of Calingasta, in the southwest of the province, from the confluence of three main rivers: the Calingasta River, the Castaño Viejo River, and the Río de los Patos. These rivers are born at an altitude of about above mean sea level in the west and southwest of the province (in contrast, the Jáchal receives its Andean tributaries from the north of the province).
From Calingasta the river flows eastwards feeding the Los Caracoles, Punta Negra and Quebrada de Ullum Dams (Embalse Quebrada de Ullum) located near San Juan city, from which it continues south-eastwards receiving a few more tributaries, including the Mendoza River, to later join the Desaguadero River near the Salina Pampa de las Latas (or Salina Bermejo), at the common border of the provinces of San Juan, Mendoza and San Luis.
The river has an average flow of . Its course is long, and its drainage basin covers an area of . The Ullum Dam is AMSL with a reservoir covering an area of and a volume of around .
The upper course of the river is used for water sports such as rafting and fishing, and windsurfing and swimming at the Ullum Dam. Along its entire length, the San Juan River is heavily used for irrigation.