Allen Parish is a parish located in the U.S. state of Louisiana. As of the 2020 census, the population was 22,750. The parish seat is Oberlin and the largest city is Oakdale. Allen Parish is in southwestern Louisiana, southwest of Alexandria. Allen Parish is named for former Confederate States Army general and Governor of Louisiana Henry Watkins Allen. It was separated in 1912 from the larger Calcasieu Parish to the southwest.