Oxalis sect. Holophyllum (Oxalidaceae) is a small group of species that are endemic to the Brazilian Atlantic Forest in the states of Rio de Janeiro, Minas Gerais, Espírito Santo, and Bahia. Species of the section can be recognized by the unifoliolate leaves, one-seeded carpels, and the common presence of dorsiventrally flattened (and sometimes winged) peduncles and/or petioles. Circumscriptions of the currently accepted species needed to be revaluated, as the overlap in several features have blurred limits among morphologically similar taxa. In this study we evaluate the monophyly and phylogenetic relationships among species of O. sect. Holophyllum, and provide an updated taxonomic study of the section, aiming at improving species circumscriptions and facilitating species-level identification. For this task, we analyzed samples from herbaria and carried out fieldwork to observe the plants in loco, collect samples, and gather material for DNA extraction, amplification, and sequencing. Bayesian and Maximum Likelihood phylogenetic analyses were performed using sequences from three markers: nrITS, and plastid trnL-trnF, and petA-psbJ from 13 of the 17 currently accepted species, which corroborate the monophyly of the section. Among the species of O. sect. Holophyllum, ten are newly described here at species level: O. adpressipila Fiaschi & Bilk (based on O. kuhlmannii Lourteig var. adpressipila Lourteig), O. capixaba Fiaschi & Bilk, O. cornicarpa Fiaschi, O. inopinata Fiaschi & Bilk, O. itamarajuensis Fiaschi & Bilk (based on O. alata Mart. ex Zucc. var. hirta Lourteig), O. leonii Fiaschi & Bilk, O. occulta Fiaschi & Bilk, O. retrorsa Fiaschi & Bilk, O. sciophila Fiaschi & Bilk, and O. septentrionalis Fiaschi & Bilk. Lectotypes are chosen for nine names, and an epitype for O. impatiens Vell. An identification key to all species is provided, as well as descriptions, illustrations, geographic distribution maps, and preliminary conservation statuses following IUCN guidelines.