This species is characterized by its narrowly obovate to oblanceolate, shortly petiolate, well developed leaves with relatively numerous secondary veins (20--25 pairs), pyramidal multiflowered inflorescences, 5-merous flowers, tubular corollas with tubes 4--5 mm long and broadly rounded lobes 0.5--1 mm long, and distinctive oblate capsules with stipitate bases. The specimens generally dry with a brown or greenish brown color. The plants appear to be at least sometimes deciduous. The capsules are generally woody. The corolla lobes measurements given by Steyermark (1974) for this species seem overly long, in general and in comparison to the overall corolla length and corolla tube lengths given there.
Simira longifolia is similar to Simira pisoniiformis, which is sympatric in the Amazon basin and has thin-textured capsules and leaves with often fewer secondary veins. Vegetatively, Simira longifolia is similar to Alseis.