The diet of the Brazilian slender opossum (Marmosops paulensis) was determined through analysis of fecal samples collected in an area of montane Atlantic forest in southeastern Brazil. M. paulensis consumed insects, arachnids, gastropods, fruits, flower parts, and small vertebrates. The observed preference for Piper fruits could be related to the spatiotemporal predictability of this item, whereas the observed preference for the Coleoptera, the Blattodea, and the Opiliones could be due to overestimation of hard-bodied prey. Diet composition varied seasonally, with invertebrates and flower parts being consumed more during the dry season, probably because of their higher relative abundance when compared to fruits in this season. Diet was more diverse during the dry season, because of the inclusion of new fruit species and the reduced contribution of Piper fruits in the diet. We suggest that M. paulensis is more frugivorous than previously thought and that this species can be an important seed disperser of pioneer plants, especially of Piper species.