San Juan is the capital and largest city of the Argentine province of San Juan in the Cuyo region, located in the Tulúm Valley, west of the Sa…San Juan is the capital and largest city of the Argentine province of San Juan in the Cuyo region, located in the Tulúm Valley, west of the San Juan River, at 650 m above mean sea level, with a population of around 112,000 as per the 2001 census.