San Acacio is an unincorporated community and a census-designated place (CDP) located in and governed by Costilla County, Colorado, United States. The population of the San Acacio CDP was 56 at the 2020 United States census. The Sanford post office serves San Acacio postal addresses.

Costilla County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 3,499. The county seat is San Luis, the oldest continuously occupied town in Colorado.