Franklin County is a county in the U.S. state of Ohio. As of the 2020 census, the population was 1,323,807, making it the most populous county in Ohio. Most of its land area is taken up by its county seat, Columbus, the state capital and most populous city in Ohio. The county was established on April 30, 1803, less than two months after Ohio became a state, and was named after Benjamin Franklin. Originally, Franklin County extended north to Lake Erie before it was subdivided into smaller counties. Franklin County is the central county of the Columbus, Ohio Metropolitan Statistical Area.

Columbus is the capital and most populous city of the U.S. state of Ohio. With a 2020 census population of 905,748, it is the 14th-most populous city in the U.S., the second-most populous city in the Midwest after Chicago, and the third-most populous U.S. state capital after Phoenix, Arizona and Austin, Texas. Columbus is the seat of government of Franklin County; it also extends into Delaware and Fairfield counties. It is the core city of the Columbus metropolitan area, which encompasses ten counties in central Ohio. It had a population of 2,138,926 in 2020, making it the largest metropolitan area entirely in Ohio and 32nd-largest metro area in the U.S.