Rockcastle County is a county located in the southeastern part of the U.S. state of Kentucky. As of the 2020 census, the population was 16,037. Its county seat is Mt. Vernon. The county was founded in 1810 and named for the Rockcastle River, which runs through it. The river, in turn, is named for its majestic rock cliffs. Rockcastle County is part of the Richmond-Berea, KY Micropolitan Statistical Area, which is also included in the Lexington-Fayette–Richmond–Frankfort, KY combined statistical area.

Mount Vernon is a home rule-class city and the seat of Rockcastle County in southeastern Kentucky. The intersection of U.S. Routes 25 and 150 is located here. The population was 2,477 at the time of the 2010 U.S. census. Mount Vernon is part of the Richmond-Berea micropolitan area.