Ruellia capotyra is described as a new species of Acanthaceae from the Atlantic Forest of Brazil, occurring in the states of Rio de Janeiro and Minas Gerais. This species was discovered during fieldwork in a natural forested area, and further details regarding its ecology and broader geographical distribution were clarified through the study of existing herbarium collections. Ruellia capotyra is a subshrub characterized by long-petiolate leaves, long-pedunculate, axillary, and compound dichasia, and rose-colored, long, and curved corollas. It differs from other species of Ruellia by its quadrangular, glabrescent branches and rachises, the absence of bracteoles, calyces with subulate, unequal lobes, and sparse, long-glandular trichomes. The new species is described and illustrated. Diagnostic characters and comments regarding its geographical distribution, phenology, and taxonomy are presented. Ruellia capotyra is compared with other species previously described in the genus and its infrageneric position is discussed. One of these species is Ruellia subsessilis, for which a lectotype is here designated and its authority corrected. An informal IUCN conservation assessment of the new species indicated that it should be considered as a threatened species, further highlighting the conservation significance of the Atlantic Forest as a whole.