During a taxonomic and phylogenetic study of Xylopia from the Amazon River basin, South America, we examined collections from the Brazilian states of Amazonas, Mato Grosso, Pará, Rondônia, and Tocantins that resembled Xylopia nitida by some characteristics of the leaves, but differed consistently from the latter in a number of floral and fruit characters. On the basis of these specimens we describe here a new species, Xylopia maasiana. The new species is a small to medium-sized tree encountered mostly in riparian habitats across the southern Brazilian Amazon and in some transition areas between the Amazon forest and cerrado biomes in Central Brazil. In addition to the detailed description, we include a preliminary assessment of conservation status, taxonomic comments, illustrations, a distribution map of the new species, and comparison with X. nitida.