Bremekamp (1963) described Trigonopyren as a genus closely related to Psychotria with nine newly described species, one in the Comores and the others in Madagascar. Trigonopyren was separated from Psychotria by its pyrene and seed characters, with the endosperm entire (i.e., not ruminate and without an adaxial invagination) and the pyrene smooth adaxially and trigonous in cross-section. He noted that the species included in Trigonopyren appear to be closely related to each other, and are characterized by costate internodes; opposite leaves that lack both pubescent and crypt-type domatia; interpetiolar, 2-lobed stipules that are deciduous through fragmentation with the persistent basal portion becoming marcescent or indurated; terminal, pedunculate, cymose, several- to multiflowered inflorescences with reduced bracts; sessile to pedicellate, 4-5-merous, monomorphic flowers with the stigmas exserted and the anthers partially exserted; small calyx limbs; small white corollas that often are pubescent at the stamen insertion; and red ovoid fruits with the characteristic pyrenes and seeds. These species also have a generally green or grayish green drying color. Bremekamp described the Trigonopyren species also as glabrous on the vegetation parts and glabrous to densely hirtellous on the inflorescence; however some recent collections of Trigonopyren are densely hirtellous throughout, and this may represent additional variation in known species or possibly undescribed species. The inflorescences are generally corymbiform, with the primary axis shorter than the secondary axes, and the inflorescence axes if various orders are generally verticillate. The pyrenes were characterized by Bremekamp as three-angled in cross-section, however those of several species vary from having three very broad angles to essentially smooth. The stipules characteristically are mostly deciduous, leaving only a thin persistent basal portion and exposing persistent, dense, ferrugineous, pilose or hirsute pubescence that is borne between the stipule and the stem. Bremekamp noted the somewhat unusual morphology of the monomorphic flowers, however he had generally 1-3 specimens of all but one species, some of which were fruiting not flowering, so his sample survey is small; the flowers of his Trigonopyren species seem to have been collected very rarely, though without the fruits they may not have been matched to these species. In particular the Trigonopyren species are similar to the species of Bremekamp's Psychotria Groups V and VI, but the Trigonopyren species can generally be recognized by their stipules with the persistent base and persistent hirsute-pilose pubescence. Bremekamp separated the species of Trigonopyren mainly by leaf characters and inflorescence size (calcualted by him as number of flowers, not length of axes or degree of branching). However there appears to be more variation in inflorescence size than he knew from the few collections he saw of several species. Habitat also seems to be important.
Subsequently Razafimandimbison et al. (2014) studied the Psychotria and related genera in the Indian Ocean region with molecular data, and found that Bremekamp's Trigonopyren included a set of species are are closely related to other Indian Ocean species that were classified in Psychotria, and this entire group of species is a part of the pantropical genus Psychotria as it is generally circumscribed. No clearly separated groups that can be characterized by exclusive geographic range or morphological characters were separated within pantropical Psycohtria, and Razafimandimbison et al. adopted a broad circumscription of this genus that includes Trigonopyren.