Pasco is a city in, and the county seat of, Franklin County, Washington, United States.Pasco is one of three cities that make up the Tri-Cities region of the state of Washington. The Tri-Cities is a mid-sized metropolitan area of approximately 279,116 people that also includes the cities of Kennewick and Richland in a 2015 estimate.Pasco's population was 59,781 at the 2010 census and 70,560 as of the April 1, 2016 Washington State Population estimate.