A new species of the didelphid marsupial genus Monodelphis is described from southeastern Brazil. The new species is closely related to Monodelphis kunsi Pine, 1975, and other members of the M. adusta group, but differs from those species and from other congeneric taxa by unique external and cranial characters and by cytochrome-b and nuclear DNA sequences. Diagnostic morphological characters of the new species include uniformly brownish dorsolateral pelage without distinct stripes or other sharp pigment discontinuities; diminutive eyes and ears; a remarkably long and narrow rostrum; apparent loss of an interparietal ossification; and the presence of diastemata between C1 and P1, P1 and P2, c1 and p1, p1 and p2, and p2 and p3. The new species is known from the Atlantic Forest (Mata Atlântica) of southeastern Brazil, where it occurs sympatrically with four other congeners (M. americana, M. iheringi, M. scalops, M. dimidiata) and seems to be restricted to montane habitats.