René Marx Dormoy (1 August 1888—26 July 1941) was a French socialist politician, noted for his opposition to the far right. Under his leadership as Minister of the Interior in the government of Léon Blum, the French police infiltrated La Cagoule, which was planning the overthrow of the French Third Republic, led by the Popular Front government. Dormoy directed the arrest and imprisonment of 70 cagoulards in November 1937. The police recovered 2 tons of armaments from their sites.
After the Occupation of France, Dormoy as a representative refused to approve providing full powers to Marshal Philippe Petain and the Vichy government. He was arrested in 1940 and interned in house arrest in Montelimar. He was assassinated there in July 1941 by a bomb set off at his house. It was believed to be the work of La Cagoule terrorists.
Born in Montluçon, René Marx Dormoy (called Marx) attended local schools and became active in politics. He was elected mayor of his native town in 1926. He was elected in 1931 as representative of the Section française de l'Internationale ouvrière (SFIO, the Socialist Party of today) to the French National Assembly from the Allier département.
A member of the Popular Front's government, cabinet secretary to Léon Blum, he played a part in negotiating the Matignon Accords. From 1936 to 1938 he was Minister of the Interior (replacing Roger Salengro). He worked to suppress violent far right groups such as the Cagoule. He used his authority to depose Jacques Doriot, mayor of Saint-Denis, arguing that the Saint-Denis commune had become the site of anti-republican agitation. He opposed illegal immigration of Eastern European political refugees to France.