The plant Mollinedia stenophylla Perkins (Monimiaceae) is endemic to southern Brazil and, until 2015, had not been seen for 122 years. We located a single population of the species on the margin of a watercourse in the mountainous region of Rio de Janeiro state. We describe the location of the species, comment on its morphology, ecology and conservation, assess its conservation status, propose conservation measures, and discuss the potential value of local action plans for this and other narrow endemic plant species. Mollinedia stenophylla has whorled leaves, clonal reproduction and a low height, unusual traits in the genus Mollinedia. Using the IUCN Red List criteria we assess the species as Critically Endangered. This example highlights the importance of investing time in plant surveys and taxonomy, especially in megadiverse countries such as Brazil.