The 21st Reserve Division (21. Reserve-Division) was a unit of the Imperial German Army in World War I. The division was formed on mobilization of the German Army in August 1914 as part of XVIII Reserve Corps. The division was disbanded in 1919 during the demobilization of the German Army after World War I. The division was raised primarily in the Prussian Province of Hesse-Nassau, but one battalion of the 88th Reserve Infantry Regiment came from the Grand Duchy of Hesse and some other troops of the division came from Westphalia and the Rhine Province.