Julien Lahaut (6 September 1884 – 18 August 1950) was a Belgian politician and communist activist. He became leader of the Communist Party of Belgium after the First World War. A dissident during the German occupation of 1940–44, he became a vocal advocate for the abolition of the Belgian monarchy during the post-war "Royal Question". His assassination in August 1950, at the height of the crisis, has often been attributed to Belgian royalists but remains unsolved.