The limits of Mostacillastrum are expanded to include 12 additional species from South America, Haiti, Mexico, and southwestern United States. A new species, M. saltaensis (Argentina) is described, and a new name, M. subauriculatum, and new combinations, M. arcuatum, M. ekmanii, M. haitiense, M. incisum, M. longipes, M. pandurifolium, M. purpusii, M. retrofractum, M. vaseyi, and M. versicolor, are proposed. A key to all 30 species of Mostacillastrum is presented.