Physeterostemon thomasii is described from the southern Bahian Atlantic forest of Brazil. It differs from the other two species of the genus by the paleaceous trichomes on young stems and petioles, and by the strongly bullate/foveolate leaves. A phylogenetic analysis with all three species, based on rbcL and nahF DNA sequence data, showed that Physeterostemon forms a monophyletic group, sister to Eriocnema fulva, and this clade is placed in a polytomy at the base of the tribe Miconieae sensu stricto. However, there is no resolution within Physeterostemon, thus the phylogenetic relationships of the three species cannot be established.