Washington County is a county in the U.S. state of Minnesota. As of the 2020 census, the population was 267,568, making it the fifth-most populous county in Minnesota. Its county seat is Stillwater. The largest city in the county is Woodbury, the eighth-largest city in Minnesota and the fourth-largest Twin Cities suburb. Washington County is included in the Minneapolis-St. Paul-Bloomington, MN-WI Metropolitan Statistical Area.