Dayton is a city in and the county seat of Columbia County, Washington, United States. The population was 2,526 at the 2010 census.
Dayton was founded in the 1860s. A town site plat was filed by Jesse N. and Elizabeth Day on November 23, 1871. The town was officially incorporated by Jesse Day on November 10, 1881. The name is in honor of Mr. Day. Dayton boasts the oldest train depot (1881) in Washington State as well as the oldest continuously used courthouse (1887).
Dayton is located at 46°19′11″N 117°58′40″W / 46.31972°N 117.97778°W / 46.31972; -117.97778 (46.319608, -117.977699).