Generic circumscriptions in the mostly pantropical family Simaroubaceae are somewhat controversial. Simaba is the largest genus, currently defined as exclusively neotropical, with around 25 species of trees and shrubs, but both its limits and infrageneric classification have been a matter of discussion and divergence. Traditionally, species of the genus have been treated in three sections: Simaba sect. Tenuiflorae, S. sect. Floribundae and S. sect. Grandiflorae, but a phylogenetic analysis suggested that the latter two may not be monophyletic. To test the monophyly of Simaba and its infrageneric classification, we used a molecular approach based on DNA sequence data from two nuclear ribosomal spacer regions (ITS and ETS) and three plastid regions (rps16 intron, and intergenic spacers psbA-trnH and trnL-trnF), including a comprehensive sampling of species from Simaba and closely related genera. We also performed ancestral character reconstructions to identify morphological characters that could serve as synapomorphies for major clades and to explore patterns of homoplasy in the morphological dataset. Our results show Simaba as traditionally circumscribed is not monophyletic, with taxa segregated into two strongly supported but distinct clades, one of which is more closely related to Simarouba. The three main clades that emerged in the phylogeny include a mostly Amazonian Simaba clade (which includes the type species of Simaba and the remaining species of S. sect. Tenuiflorae, here proposed to be recognized as Simaba sensu stricto), a mostly extra-Amazonian Simaba clade (a distinct lineage that will be recognized as Homalolepis, a genus currently treated in synonymy and equivalent to Simaba sections Grandiflorae and Floribundae), and the Simarouba clade (including all of its current species). These three clades are characterized by a combination of morphological characters, described in detail herein, some of which are novel features for Simaba not previously reported in the literature. Mapping character-states on the phylogenetic tree provides tests for evolutionary hypotheses. For example, our reconstruction of habit and geographic distribution suggests that the diversification of several shrubby species within the extra-Amazonian lineage in the South American cerrados probably occurred from ancestors inhabiting tropical forests, involving transitions in morphological and ecological traits.