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 230,000 people that also includes the cities of Kennewick and Richland. The population was 32,066 at the 2000 census. Since that time the city has experienced rapid growth, and the official April 1, 2009 estimate from the Washington State Office of Financial Management puts the city's population at 54,490.