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is a Statutory Town in, the county seat of, and the most populous town of Costilla County, Colorado, United States. Officially established on 1851-04-09 by Hispanic settlers from Taos, New Mexico, San Luis is the oldest town in the state of Colorado. The population was 739 at the 2000 census.
The town of San Luis lies within the San Luis Valley and is located at (37.201988, -105.422360).
According to the United States Census Bureau, the town has a total area of 0.5 square miles (1.2 km²), all of it land.