There are 3 US Post Offices in Adams County serving a total of 20,160 residents equating to 6,720 residents per post office. It's estimated that approximately 32,178 packages are handled across the 3 county post offices annually. Adams County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 18,728. The county seat is at Ritzville, and its largest city is Othello. The county was formed out of Whitman County on November 28, 1883 and is named for the second President of the United States, John Adams. Adams County comprises the Othello, WA Micropolitan Statistical Area, which is also included in the Moses Lake-Othello, WA Combined Statistical Area."