(In French with some English) During the last 20 years of his life, the art dealer Hayashi Tadamasa (1853-1906) received important and impassioned correspondence from around the world. This specialist in Japanese art, connoisseur of Western art, and friend of many artists, collectors and intellectuals, is revealed through these letters, which assist in determining the fundamental role played by Hayashi Tadamasa in the West and Japan.