Cassville is a city in Flat Creek Township, Barry County, Missouri, United States. According to the 2020 census, the population of Cassville wa…Cassville is a city in Flat Creek Township, Barry County, Missouri, United States. According to the 2020 census, the population of Cassville was 3,190. Cassville is one of the primary markets and job centers for an estimated 14,000 people living in the surrounding area. It is the county seat of Barry County.