Karl Loewenstein (November 9, 1891 in Munich – July 10, 1973 in Heidelberg) was a German lawyer and political scientist, regarded as one of the prominent figures of Constitutional law in the twentieth century.
His research and investigations into the typology of the different constitutions have had some impact on the Western constitutional thought. Loewenstein is credited with establishing the theoretical foundations of militant democracy to battle anti-democratic mass movements.
He worked as lecturer in his native city of Munich when Adolf Hitler's Nazi Party took power in 1933. Loewenstein went to the United States, becoming chair of political science at Amherst College and published extensively. He received American citizenship and after World War II worked for the Allied Military Government of Occupied Territories (AMG) in West Germany.