Old Jefferson is a census-designated place (CDP) in East Baton Rouge Parish, Louisiana, United States. The population was 6,980 at the 2010 census, up from 5,631 in 2000. It is part of the Baton Rouge Metropolitan Statistical Area.

East Baton Rouge Parish is the most populous parish in the U.S. state of Louisiana. Its population was 456,781 at the 2020 census. The parish seat is Baton Rouge, Louisiana's state capital. East Baton Rouge Parish is located within the Greater Baton Rouge area.