This species is characterized by its reduced or deciduous pubescence, medium-sized rather narrowly obovate leaves, stipules 3-5 mm long, pedicillate flowers, calyx lobes ca. 1 mm long, corollas ca. 3 mm long, and slender capsules ca. 5 x 1 mm. The inflorescences axes are slender and somewhat flexuous, and quite elongated on the few specimens seen.

The protologue described this species as having 6 calyx and corolla lobes; the number of anthers was not noted but is 5 on the flowers on the specimen imaged at R. The resolution on this image is not adequate to count the corolla lobes, but many of the flowers on this specimen do have an evidently 6-lobed calyx limbs. Some flowers, however, appear to have 5 or 7 calyx lobes, so in general this may be a variant plant of this species or this species may be unusually variable in Alseis. Such variation is found in occasional species of Rubiaceae, and more often on individual flowers on a single plant. Bremekamp then described his Alseis longifolia var. pentamera based on a plant from Surinam with the calyx and corolla 5-lobed, and such infraspecirfic variation would not be unexpected in Alseis. 

Lowland wet forest in northeastern Brazil (Amapá, Pará) and in Suriname. Bremekamp noted in the protologue of Alseis longifolia var. pentamera that Alseis longifolia was also reported from Guayana by Sandwith in Kew Bull. 1933: 333, but the specimen cited there has not been seen and may belong instead to Alseis smithii, which was described subsequently.