Fort DeRussy, Civil War fort in Marksville, United States.

Fort DeRussy is a Confederate earthwork fort built in 1862 south of Alexandria and four miles north of Marksville, Louisiana. Its surviving earth walls and water batteries once guarded the lower Red River Valley. The site, in Avoyelles Parish, also holds a cemetery and was designated a state historic site in 1994.

Built in 1862 to defend the lower Red River Valley, the fort was named for Colonel Lewis G. DeRussy, the engineering officer in charge of its first fortifications. Part of Rear Admiral David D. Porter's Mississippi Squadron captured it on May 5, 1863, and the USS Benton destroyed areas on May 9. Its remaining garrison surrendered to General A. J. Smith's Union troops on March 14, 1864.

The fort is named for Colonel Lewis G. DeRussy, a West Point graduate and engineer who designed its first fortifications. His remains were exhumed from an abandoned grave and reinterred on the fort grounds on September 26, 1999. The site also includes a cemetery and water batteries.

The site is located a few miles north of Marksville and is accessible by road, with walking paths through the earthworks. The cooler months offer more comfortable visiting conditions, as summer heat and humidity in the area can be significant.

Colonel Lewis G. DeRussy, the fort's namesake, died of a heart attack at his plantation home in December 1864; his remains were exhumed from an abandoned grave and reinterred on the fort grounds on September 26, 1999.