Middleburg is an unincorporated area and census-designated place (CDP) within Clay County in the U.S. state of Florida, located 26 miles (42 km) southwest of downtown Jacksonville and 16 miles (26 km) northwest of Green Cove Springs, the county seat of Clay County. As of the 2020 census, the population of Middleburg was 12,881.

Clay County is located in the northeastern part of the U.S. state of Florida along the west bank of the St. Johns River. As of 2020, the population was 218,245 and in 2023, that number increased to 232,439, making it the third largest county in the Jacksonville metropolitan area. While most of the county is unincorporated, there are 4 municipalities with Green Cove Springs being the county seat and the unincorporated Lakeside CDP being the largest place. It is named in honor of Henry Clay, a famous American statesman, member of the United States Senate from Kentucky, and United States Secretary of State in the 19th century.