This species is characterized by its robust habit; brown drying color; generally elliptic to obovate leaves with numerous secondary veins, a usually well developed submarginal vein, and no domatia; subsessile to shortly pedunculate, congested-cymose inflorescences with reduced bracts; subsessile flowers with the calyx limb 3-5 mm long and a reddish orange to perhaps yellow corolla; and large subglobodse fruits 15-20 mm in diameter with stipules or pedicels elongating as they mature. Bremekamp noted in the protologue that the type was described as having coral red corollas but that was probably an error; however Birkinshaw 20 also describes orange-red corollas. The mature fruit color is not noted by collectors and may be green, as apparently is the case the other Psychotria species in Madagascar with similarly large fruits. The fruits of Psychotria nossibensis are noted by Antilahimena 228 and 451 to be eaten by the black lemur. Psychotria nossibensis is similar in its large leaves and fruits to Psychotria megalocarpa of easstern Madagascar, but Psychotria megalocarpa has generally larger laxer inflorescences and leaves without a well developed submarginal vein.

Bremekamp recognized two varieties of this species, which he distinguished by a single character: var. nossibensis has the disk portion of the flower markedly prolonged on the fruits, while var. arrhyncha lacks this prolongation of the disk. This prolonged disk appears to be unique within Psychotria, and little else is known about it. The material of this species is too limited at present to evaluate this character further.