This study represents the first monograph of Colombian species of the neotropical and strictly montane genus Monochaetum based on morphology. Colombia stands out as a major centre of diversity for neotropical Melastomataceae and represents the richest regional centre for the genus Monochaetum, with 27 species. An evaluation of species limits in the South American region with an emphasis on the species occurring in Colombia is presented by incorporating and integrating information from macromorphology, trichome, pollen, and seed micromorphology, along with a summary of known chromosome numbers. The nomenclature was reviewed for each taxon. Full synonymies and descriptions are provided, as well as a key. Each species is mapped, and most are illustrated. New synonymies are proposed (eight species, three varieties, two subspecies), and one new species, Monochaetum tonaense, is described.