Union County is the northeasternmost county in the U.S. state of New Mexico. As of the 2020 census, the population was 4,079, making it the fourth-least populous county in New Mexico. Its county seat is Clayton. The county was formed in 1894. Union County borders Colorado to the north, and Oklahoma and Texas to the east.

Clayton is a town and county seat of Union County, New Mexico, United States. As of the 2010 census, the city population was 2,980.