Washington County is a county located in the northeastern part of the U.S. state of Oklahoma. As of the 2020 census, the population was 52,455. Its county seat is Bartlesville. Named for President George Washington, it is the smallest county in Oklahoma in total area, adjacent to the largest county in Oklahoma, Osage County.

Bartlesville is a city mostly in Washington County and Osage County, Oklahoma. The population was 37,290 at the 2020 census. Bartlesville is 47 miles (76 km) north of Tulsa and 18 miles (29 km) south of the Kansas border. It is the county seat of Washington County. The Caney River runs through Bartlesville.