Mentzelia sect. Bartonia (Loasaceae) is the most species-rich of the six sections of Mentzelia. The section consists of overwintering annuals, biennials, and short- to long-lived, sometimes subshrubby perennials whose geographic ranges are centred primarily in western North America. Synapomorphies that support its monophyly include winged seeds that are horizontally positioned in capsules, seed testal microsculpture consisting of straight, wavy, or sinuate anticlinal cell walls with 4–106 papillae per cell, and broadened filaments (without distal flanges) on the outermost stamens positioned opposite the sepal lobes. Monophyly of the section is well supported by molecular characters from plastid and nuclear DNA. Despite the distinctiveness of species in sect. Bartonia from those of other sections, patterns of variation within the section have led to taxonomic difficulties that have obscured our understanding of diversity within the section. We applied a phylogenetic approach with DNA sequence data, sampled widely across geographic and taxic entities, including multiple populations of putatively morphologically variable and geographically disjunct species, to assess partitioning of morphological diversity among lineages in sect. Bartonia. The authors identified two major lineages that diverged early in sect. Bartonia. The first lineage, clade 1, includes M. multiflora, M. humilis, M. pumila, M. laevicaulis, and relatives. The second lineage, clade 2, includes M. decapetala, M. multicaulis, M. argillosa, and relatives, many of which are subshrubby, short-lived perennial species centred geographically in the intermountain West. The lineages we reconstructed served as a basis for taxonomic revision, and our results are consistent with the recognition of 51 species, including three with varieties. This number of species is nearly twice that recognized in earlier Mentzelia monographs. Types are reviewed, and names are lectotypified as necessary. Character synopses, including taxonomically critical seed microsculpture data and floral petal and stamen shape series, distribution maps, conservation status, and taxonomic keys (including a separate key to the eight taxa that occur in Mexico) are provided for the species and varieties.