South American taxa of Mentzelia L. (Loasaceae) are revised and morphologically and ecologically characterized on the basis of extensive field studies, cultivation of several taxa, and a revision of over 1000 herbarium specimens (over 550 collections). On the basis of this study, a total of 14 taxa in eight species and six subspecies are recognized: M. albescens (Gillies ex Arn.) Griseb., M. aspera L., M. bartonioides (C. Presl) Urb. & Gilg, M. heterosepala Weigend & E. Rodr., M. parvifolia Urb. & Gilg ex Kurtz, and M. scabra Kunth, plus the two new species M. sericea Weigend (northern Peru) and M. angurate Weigend (central Peru). The latter are segregated from M. scabra, which is redefined. Mentzelia scabra is subdivided into six subspecies: one represents the autonym (subsp. scabra, Colombia), four are new status and combinations (subsp. cordobensis (Urb. & Gilg ex Kurtz) Weigend [Argentina], subsp. grandiflora (Ruiz & Pav.) Weigend [coastal Peru], subsp. atacamensis (Urb. & Gilg) Weigend [northern Chile], subsp. chilensis (Gay) Weigend [Venezuela, Colombia, Ecuador, Peru, Chile, Bolivia]), and one is a new subspecies (subsp. boliviana Weigend [Bolivia]). Lectotypes are designated for M. andina Urb. & Gilg, M. cordifolia Dombey ex Urb. & Gilg, M. fragilis Huber, M. grisebachii Urb. & Gilg, M. parvifolia, M. scabra subsp. atacamensis, M. scabra subsp. chilensis, M. soratensis Urb. & Gilg, and M. scabra subsp. cordobensis. The following taxa are newly synonymized: M. pinnatifida (Phil.) Urb. & Gilg (= M. bartonioides), M. jujuyensis Sleumer and M. parvifolia var. transiens Sleumer (= M. parvifolia), M. cordifolia (= M. scabra subsp. grandiflora), and M. fendleriana Urb. & Gilg, M. ignea (Phil.) Urb. & Gilg, and M. soratensis (= M. scabra subsp. chilensis). Taxon delimitations in M. scabra and its allies have traditionally been based on stamen number, petiole length, petal length, leaf subdivision, seed morphology, and density of trichome cover on stem, all of which are of limited systematic usefulness. Taxa are redefined here primarily on the basis of trichome characters on the ovary, a rather conservative (within and between populations) and readily observed character complex, and additionally on the basis of growth habit, presence or absence of persistent bark, and, at an infraspecific level, fruit shape and petal length.