Spokane County is a county located in the U.S. state of Washington. As of the 2010 census the population was 471,221, making it the fourth-most populous county in Washington state. The largest city and county seat is Spokane, the second largest city in the state, behind Seattle.Spokane County was formed on January 29, 1858. It was annexed by Stevens County on January 19, 1864, and re-created on October 30, 1879.