Adams County is a county in the U.S. state of Washington. As of the 2020 census, the population was 20,613. The county seat is at Ritzville, and its largest city is Othello. The county was formed out of Whitman County in November 1883 and is named after John Adams, the second President of the United States. Adams County's two most populous cities are Ritzville and Othello.