Middleburg is an unincorporated area and census-designated place within Clay County in the U.S. state of Florida, located 26 miles southwest of downtown Jacksonville and 16 miles northwest of Green Cove Springs, the county seat of Clay County. As of the 2020 census, the population of Middleburg was 12,881, down from 13,008 at the 2010 census. It is part of the Jacksonville, Florida Metropolitan Statistical Area. It is home to Middleburg High School and to the Middleburg Historic District.