DeRidder is a small city in and the parish seat of Beauregard Parish, Louisiana, United States. A small portion of the city extends into Vernon Parish. As of the 2010 census DeRidder had a population of 10,578. It is the smaller principal city of the Fort Polk South-DeRidder CSA, a Combined Statistical Area that includes the Fort Polk South (Vernon Parish) and DeRidder (Beauregard Parish) micropolitan areas, which had a combined population of 87,988 at the 2010 census.