This species is characterized by its rather robust, glabrous to infrequently hirtellous or pilosulous habit; its generally well developed, tubular to spathaceous, truncate to obtuse stipules that are sometimes 2-denticulate; its stiff, shiny, geneally obovate (to elliptic or oblanceolate) leaves with the venation well developed and usually (though not always) with a few small domatia; its corymbiform, pedunculate inflorescences with the spreading secondary axes longer than the primary axis and often curved; its small yellow flowers that are subsessile in congested cymes; and its red, subglobose, somewhat large fruits. This is one of the most commonly collected Psychotria species in Madagascar, and as can be expected it shows variation in several features including leaf size, inflorescence size, calyx limb length, and drying color. However the stipules are distinctive and tubular to spathaceous. In bud they form a conical structure with the intrapetiolar sides each apparently folded in toward the bud; as this part of the stem matures the stipules open into a tube, and then frequently split shortly to deeply along one side to form a triangular upper portion and a sheathing base. The triangular upper portion of the stipules sometimes appears to be an interpetiolar stipule, but in the stipules examined only one lobe was seen on the entire structure, or sometimes this lobe then split for up to 0.5 mm. The stipules are various glabrous to densely hirtellous externally (i.e., abaxially). Bremekamp described the stipules of Psychotria parkeri as 4-8 mm long, but on some specimens these range to 12 mm long (e.g., Ralimanana & Ranaivojaona 209, det. K; Antilahimena et al. 6015), especially on vegetative apices. Bremekamp also characterized Psychotria parkeri as having domatia on the leaves, and these are characteristically present but may be few and small, and then can be overlooked. Psychotria parkeri has been recognized in part in practice by its yellow-green drying color, however many collections treated with alcohol lack this.

Psychotria parkeri is similar to Psychotria reducta, and these have often been confused and may not be distinct. These are separated by their stipules, which are partially fused into a basal tube that often splits along one side and has the interpetiolar portion entire to 2-denticulate in Psychotria parkeri, vs. the stipules shortly fused with the basal portion not splitting or splitting on two sides and each interpetiolar portion with two well developed, slender, deciduous lobes. The leaves of Psychotria reducta are generally rather small, but they overlap in size with those of small plants of Psychotria parkeri. No other differences are evident between these two species.

Psychotria parkeri is also similar to Psychotria menalohensis, which can be separated by it inflorescences that are fastigiate or have ascending axes and ellipsoid fruits that become stipitate as they mature; the type specimen of Psychotria menalohensis is in flower, and perhaps should be re-checked when all these species are better understood for its identity vs. Psychotria parkeri. Also Psychotria parkeri has been confused with Psychotria onivensis, but Psychotria onivensis has pedicellate flowers, pedicellate to markedly stipitate fruits, and quite sharp leaf apices. Psychotria parkeri has also been confused with Psychotria imerinensis, but Psychotria imerinensis has interpetiolar stipules with two well developed narrow lobes on each interpetiolar side and the leaves generally truncate to rounded at the apex.