Gracilinanus emiliae is a species of gracile mouse opossum that is currently known from fewer than a dozen specimens. The material we examined exhibits a distinctive suite of external and craniodental characters that are unlike those of any other congeneric species. We review all published records for G. emiliae, several of which are based on misidentifications or lost material, and we report a new specimen from northeastern Peru, the first to be recorded from that country, and the first to be unequivocally associated with primary lowland rain forest. The available distributional data suggest that G. emiliae (or a complex of cryptic taxa with the same morphological characters) is very widely distributed in Amazonia and perhaps also in Venezuelan coastal rain forests. Phylogenetic analyses of morphological and molecular data suggest that G. emiliae is the sister taxon to other analyzed species of Gracilinanus, the biogeographic history of which is likely to be complex. © SAREM, 2009.