Williamstown is a home-rule-class city in Grant and Pendleton counties in the U.S. state of Kentucky. The population was 3,925 at the 2010 census, up from 3,227 as of the 2000 census. It is the county seat of Grant County.

Grant County is a county located in the northern part of the U.S. state of Kentucky. As of the 2020 census, the population was 24,941. Its county seat is Williamstown. The county was formed in 1820 and named for Colonel John Grant, who led a party of settlers in 1779 to establish Grant's Station, in today's Bourbon County, Kentucky. Grant County is included in the Cincinnati-Middletown, OH-KY-IN Metropolitan Statistical Area. Grant County residents voted to allow full alcohol sales in the county by a margin of 56% to 44% in a special election on December 22, 2015. In the 19th century, Grant County had multiple saloons.