Yuma County is a county in the southwestern corner of the U.S. state of Arizona. As of the 2020 census, its population was 203,881. The county seat is Yuma. Yuma County includes the Yuma, Arizona Metropolitan Statistical Area. The county borders three states: Sonora, Mexico, to the south, and two other states to the west, across the Colorado River: California of the United States and the Mexican state of Baja California.