The XXI Army Corps / XXI AK () was a corps level command of the German Army, before and during World War I. As the German Army expanded in the latter part of the 19th Century and early part of the 20th Century, the XXI Army Corps was set up on 1 October 1912 in Saarbrücken as the Generalkommando (headquarters) for the districts of Koblenz, Trier and part of Alsace-Lorraine. It took over command of 31st Division from XV Corps and the newly formed 42nd Division (the last division to be formed by the peacetime army). General der Infanterie Fritz von Below, former commander of 1st Guards Division, took command.