Monochilus (Lamiaceae) is an enigmatic genus endemic to Brazil with two rarely collected species currently recognized. Based on morphological features Monochilus is hypothesized to be closely related to Amasonia within subfamily Ajugoideae. However, Monochilus is one of the few genera within Lamiaceae never to be represented in a published molecular phylogenetic study, and thus has an uncertain placement with the subfamily. During recent herbarium exploration in association with the Flora of Brasil 2020 project, new specimens of Monochilus gloxinifolius and M. obovatus were uncovered. Based on this material, a new phylogeny of Ajugoideae was constructed based on nuclear ribosomal ITS and chloroplast matK and trnL-trnF markers. Monochilus, as previously hypothesized, is closely related to Amasonia and placed within Ajugoideae. We discuss the phylogenetic placement of the genus and provide a taxonomic revision with description, distribution maps, an identification key, preliminary conservation assessments, photographic plates, and comments on both the taxonomy and ecology of these two species. Additionally, we provide a lectotypification for M. gloxinifolius.