Union is a home-rule-class city in Boone County, Kentucky, United States. The population was 7,416 as of the 2020 United States Census. The area was rural until residential growth in the 1990s and 2000s. Union is located 17 miles (27 km) southwest of Cincinnati, Ohio.

Boone County is a county located on the Ohio River in the northernmost part of the Commonwealth of Kentucky. As of the 2020 census, the population was 135,968, making it the fourth-most populous county in Kentucky. Its county seat is Burlington. The county was formed in 1798 from a portion of Campbell County. and was named for frontiersman Daniel Boone. Boone County, with Kenton and Campbell Counties, is of the Northern Kentucky metro area, and the Cincinnati-Middletown, OH-KY-IN Metropolitan Statistical Area. It is the location of the Cincinnati/Northern Kentucky International Airport, which serves Cincinnati and the tri-state area.