San Juan County is a county located in the southeastern portion of the U.S. state of Utah. As of the 2010 census, the population was 14,746. Its county seat is Monticello, while its most populous city is Blanding. The county was named by the Utah State Legislature for the San Juan River, itself named by Spanish explorers (in honor of Saint John).San Juan County borders Arizona, Colorado, and New Mexico at the Four Corners.