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Abstract Spermacoceae is the largest predominantly herbaceous lineage in the Rubiaceae, and regarded as one of its most taxonomically complex and controversial tribe, especially regarding generic delimitation. Molecular phylogenies have established the concept of a broadly circumscribed Spermacoceae, which includes the genera traditionally associated with the tribe, i.e., the Spermacoce clade, and the former Hedyotideae and Manettieae. A major challenge in the Spermacoce clade is related to the delimitation of Borreria and Spermacoce , which appeared intermingled with several smaller and morphologically well‐defined genera in more recent phylogenies, such as the Brazilian endemic Psyllocarpus . One of its species, P. densifolius , diverges morphologically from the others, as preliminary phylogenetic analyses have shown that it is not closely related to Psyllocarpus nor any other genus in the Spermacoce clade. We conducted a broad phylogenetic analysis, including ca. 63% of the genera in the Spermacoceae, using a variety of approaches to address incongruence among markers and lack of resolution and/or low support for branches, to accurately assess its position. We used nuclear ribosomal (ITS) and plastid ( rps16 intron, trnL‐F intergenic spacer) DNA regions for Bayesian inference and maximum likelihood analyses, as well as species tree inference. We identified incongruence not only between nuclear and plastid DNA but also between the plastid regions. We also showed that the topologies derived from the multispecies coalescent diverged from that of the concatenated analysis, even though the position of P. densifolius , as sister to the clade formed by the remaining Spermacoceae, remained stable across different datasets and inference methods. We here propose the recognition of the new genus Diadorimia to accommodate P . densifolius , which can be characterized by its cespitose habit arising from a well‐developed, woody subterranean system, triangular stipules with margins bearing four small lobes, heterostylous flowers subtended by a pair of hyaline bracteoles, double reticulate pollen grains, and capsules with longitudinal‐oblique dehiscence, with the two valves forming one single caducous diaspore keeping their respective seed trapped inside. We provide a taxonomic treatment with a key to the heterostylous genera of the Spermacoceae occurring in the Americas, a description of the new genus, and the new combination of its type, as well as comments on its distribution and conservation.