Justin K. Thannhauser (1892–1976) was a German art dealer and an important figure in the development and dissemination of Modern art in Europe.
Justin K. Thannhauser was the son of Heinrich Thannhauser (1859–1935), who was also an art dealer. As a child, Justin assisted his father in his Moderne Galerie, one of the three Thannhauser Galleries which was located in Munich. In 1911, Heinrich sent him abroad to further his academic studies, including art history, philosophy, and psychology. Justin studied in Munich, Berlin, Florence, and Paris, with renowned scholars such as Henri Bergson, Adolf Goldschmidt, and Heinrich Wölfflin. He later brought Wölfflin and other eminent guests to hold private lectures at the Moderne Galerie, helping to turn it into one of Munich's leading art galleries. While abroad, he also strengthened business contacts with artists and other important art dealers, such as Kahnweiler (Pablo Picasso's Parisian dealer) and Wilhelm Uhde.
In 1912, Justin returned to Munich to work full-time at the gallery, which had become known for its combination of Munich Modernism (see Neue Künstlervereinigung München and Der Blaue Reiter) and French avant-garde painting (see Impressionism and Post-Impressionism). His new business relationship with Filippo Tommaso Marinetti, however, led to an unprecedented exhibition of works by the Italian Futurists. This exhibition, while controversial, brought even greater notoriety to the gallery. In 1913, it held the largest and most comprehensive exhibition of Picasso's work. Justin wrote the introduction to the exhibition catalogue and soon began a close personal and professional relationship with the artist. He acquired many of Picasso's works over the years, including Woman Ironing (1904) and The Blind Man's Meal (1903).