Fort Reynolds was a Union Army redoubt built as part of the defenses of Washington, D.C., in the American Civil War. The fort was located in Fairlington, Arlington County, Virginia. It was constructed in September 1861 to command the approach to Alexandria by the Four Mile Run valley and was itself protected by nearby Battery Garesche. It had a perimeter of 360 yards (329 m) and emplacements for 12 guns.