Léonce Rosenberg (12 September 1879, Paris – 31 July 1947, Neuilly-sur Seine) was an art historian, art collector, publisher and one of the most influential French art dealers of the 20th century. The son of an antique dealer Alexander Rosenberg and brother of the gallery owner Paul Rosenberg (21 rue de la Boétie, Paris), Léonce, a prominent gallery owner in Paris at the end of World War I, would become one of the world's major dealers of Modern art.
Leaving the family-owned gallery in 1910 Léonce opened his own business called Haute Epoque at 19 rue de La Baume, Paris. As an antiquarian Rosenberg began buying works by Cubist artists. By 1914 his collection included works by Pablo Picasso, Jean Metzinger, Albert Gleizes, Auguste Herbin, and Juan Gris. After serving in World War I (1916-1917) he pursued his interests and represented these and other artists, and by the end of the war opened a new show space, .
Léonce Rosenberg studied in London and Antwerp visiting galleries and museums in his free time. After returning to Paris he worked with his brother Paul in the family business. In 1906 Léonce and his brother inherited the family gallery on Avenue de l'Opéra which had been in existence for twenty years. His brother Paul was largely engaged in 19th- and early 20th-century art.
Léonce Rosenberg was an early advocate of Cubism, and would remain so throughout the 1920s and 1930s. He discovered the works of avant-garde artists in 1911 through the Salon des Indépendants, the art dealer Wilhelm Uhde, and in 1912 at the gallery of Daniel-Henry Kahnweiler.