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. Washington County comprises the Bartlesville, OK micropolitan statistical area, which is also included in the Tulsa-Muskogee-Bartlesville, OK combined statistical area.