Randia heteromera is a new species described based on studies of taxonomy and vegetative anatomy (leaf, stipule, and colleters). This species is illustrated and diagnosed in comparison to the other four similar species of Randia in the Southern Cone of America. The species grows in areas of humid forests, locally known as the humid Chaco in the southern part of its distribution (Argentina and Paraguay), and in seasonal forests in the northern part, in northeast Paraguay and the south of Mato Grosso do Sul in Brazil. We present a key to distinguish R. heteromera from the other species of the Southern Cone of America. The species was also compared with similar taxa, Randia nitida or R. hebecarpa, occasionally used to identify material from the Southern Cone of America. A distribution map is provided. The anatomical data studied had a taxonomic value at the species level. In addition, based on the leaf anatomy studies carried out in the present study and in previous studies, three new types of vascular tissue organization in the leaf are described for Rubiaceae.