Johnson County is a county in the U.S. state of Kansas, along the border of the state of Missouri. Its county seat is Olathe. As of the 2020 census, the population was 609,863, the most populous county in Kansas. The county was named after Thomas Johnson, a Methodist missionary who was one of the state's first settlers. Largely suburban, the county contains a number of suburbs of Kansas City, Missouri, including Overland Park, a principal city of and second most populous city in the Kansas City Metropolitan Area.

Olathe is the county seat of Johnson County, Kansas, United States. It is the fourth-most populous city in both the Kansas City metropolitan area and the state of Kansas, with a 2020 population of 141,290.