The Black Reichswehr was the unofficial name for the extra-legal paramilitary formation that was secretly a part of the German military (Reichswehr) during the early years of the Weimar Republic. It was formed in 1921 after the government banned the Freikorps that it had relied on until then to supplement the Reichswehr. General Hans von Seeckt thought that the Reichswehr no longer had enough men available to guard the country's borders, but the army could not be expanded because of the manpower restrictions imposed on it by the Treaty of Versailles. In order to circumvent the limitation, Seeckt created the Black Reichswehr as purportedly civilian "labour battalions" attached to regular Reichswehr units. The Arbeitskommandos received military training, provisioning and orders from the Reichswehr, although ultimately they were never involved in military action. The Black Reichswehr reached a peak membership estimated at 50,000 to 80,000 in 1923 and was dissolved the same year after a group of its members launched the failed Küstrin Putsch. Its existence became widely known in 1925 when its practice of Fememord, the extra-judicial killing of "traitors" among its ranks, was revealed to the public.

Reichswehr was the official name of the German armed forces during the Weimar Republic and the first years of the Third Reich. After Germany was defeated in World War I, the Imperial German Army was dissolved in order to be reshaped into a peacetime army. From it a provisional Reichswehr was formed in March 1919. Under the terms of the Treaty of Versailles, the rebuilt German Army was subject to severe limitations in size, structure and armament. The official formation of the Reichswehr took place on 1 January 1921 after the limitations had been met. The German armed forces kept the name Reichswehr until Adolf Hitler's 1935 proclamation of the "restoration of military sovereignty", at which point it became part of the new Wehrmacht.