A new species, Psidium araucanum Soares-Silva & Proença (Myrtaceae), endemic to the states of Paraná and São Paulo in south-eastern Brazil, is described and illustrated. It is most similar to Psidium robustum Mart. ex DC., which it resembles in inflorescence structure and in having medium-sized open buds and medium-sized fruits with relatively few, large seeds. Psidium araucanum is distinguished by its off-white pubescence, which is dense on new growth, and by being a large tree with membranous leaves, brochidodromous venation, long petioles and peduncles, well-developed, somewhat unequal calyx lobes, and small roundish anthers with few inconspicuous glands. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 51–54.