The identity of this species is not entirely clear. It is characterized by small, shortly bidentate stipules; well developed, petiolate, quite narrow, leathery leaves with the secondary veins not visible and without domatia; rather short corymbiform inflorescences with the axes paired on the type specimen but described in the protologue as whorled; small subsessile flowers with a short calyx limb and the corolla unknown; and small ellipsoid fruits. The type specimen of Psychotria assimilis is very similar to Psychotria elliotii, and Bremekamp distinguished these mainly by inflorescence size with 100+ flowers in the inflorescences of Psychotria elliotii vs. 20-40 in Psychotria assimilis. Here these two species are distinguished arbitrarily by range and habitat, with plants from littoral forest in southeastern Madagascar included in Psychotria elliotii and plants from higher-elevation humid forest in east-central Madagascar included in Psychotria assimilis.

Psychotria assimilis is also similar to Psychotria oocarpa, which may not be distinct. Bremekamp separated Psychotria oocarpa by its inflorescences with the axes 5-verticillate and ca. 50 flowers and its fruits "prolate" (ellipsoid-oblong), vs. the inflorescences with ternate axes and 20-40 flowers and the fruits "subglobose" in Psychotria assimilis. Neither of these species had the flowers fully documented and the fruits of the type of Psychotria assimilis can arguably also be considered ellipsoid; if fruit shape can vary to some degree within Psychotria species in Madagscsar then the separation of these two species needs reevaluation.

In addition to these taxonomic problems, the name Psychotria assimilis has been rather widely used in herbarium identifications for various other Psychotria species. In particular for plants that are here included in Psychotria lucidula, which has generated additional confusion about the identity of Psychotria assimilis.