Recent studies have shown that the genus Acacia Miller s.l. is polyphyletic, consisting of at least five distinct groups of species. One of these groups, the proposed genus Mariosousa Seigler & Ebinger, consists of 13 species. We have made the following new combinations: Mariosousa acatlensis (Bentham) Seigler & Ebinger, M. centralis (Britton & Rose) Seigler & Ebinger, M. compacta (Rose) Seigler & Ebinger, M. coulteri (Bentham in A. Gray) Seigler & Ebinger, M. dolichostachya (S. F. Blake) Seigler & Ebinger, M. durangensis (Britton & Rose) Seigler & Ebinger, M. mammifera (Schlechtendal) Seigler & Ebinger, M. millefolia (S. Watson) Seigler & Ebinger, M. russelliana (Britton & Rose) Seigler & Ebinger, M. salazarii (Britton & Rose) Seigler & Ebinger, M. sericea (Martens & Galeotii) Seigler & Ebinger, M. usumacintensis (Lundell) Seigler & Ebinger, and M. willardiana (Rose in Vasey & Rose) Seigler & Ebinger. These species are restricted to tropical and subtropical regions of the southwestern United States, Mexico, and Central America. In addition to their close geographic affinities, a series of morphological characteristics, as well as recent molecular data, separate this new genus, which has been commonly referred to as the Acacia coulteri group. They are morphologically distinct from other species of Acacia subg. Aculeiferum Vassal in that they always lack prickles and are never lianas. Although this group of species is monophyletic, previous taxonomic treatments have not dealt with them as a separate unit within Acacia subg. Aculeiferum.