This species is characterized by its tree habit; medium-sized to somewhat small, elliptic to ovate, petiolate leaves with rounded to truncate or shortly cordate bases and about 7-12 pairs of secondary veins; shortly pedunculate, subcapitate to congested-cymose, several-flowered, bracteate inflorescences; pedicellate 5-merous flowers; lobed calyx limbs 2-5 mm long; tubular to urceolate-funnelform, greenish yellow or pale green, medium-sized (for Simira) corollas, and subglobose woody capsules 2.5-3 cm in diameter. The specimens generally dry brownish green to rather dark brown or occasionally are reddish purple throughout. Individual plants vary in pubescence from glabrous (except for the domatia) to sparsely to moderately strigillose or strigose on the leaves and densely hirtellous on petioles and stems. The inflorescences initially are subsessile and subcapitate, and enclosed by 1-2 pairs of stipules 10-15 mm long; as the inflorescences develop, the enclosing stipules spread and then fall off, the peduncle elongate, and the axes and pedicels elongate. The mature inflorescences are not lax, and are hemispherical to subglobose in overall form. The bracts are regularly developed, triangular, acute, and 3-5 mm long, and deciduous. The anthers and stigmas are well exserted from the corolla, and the styles and stigmas appear on the dried specimens to be curved and probably held off on one side of the flower. The capsules are generally rather thinly woody, to almost chartaceoous, and on dried specimens frequently have a large split into two halves, and then these split again to produce four valves; whether this is an artifact of drying is not clear. Simira rubescens is the most widespread and commonly collected species of this genus.
This species is widespread and commonly collected, and shows some variation across its range. Plants from the western part of the range rather consistently have glabrous leavs with regularly developed domatia, calyx limbs 2-3 mm long with rounded lobes and corollas with tubes 4-5 mm long. Plants from the central and eastern Amazon basin vary more, with the leaves glabrous to moderately strigillose or hirtellous with small domatia, calyx limbs 4-5 mm long that are shallowly lobed to divided nearly to the base into ovate lobes, and corollas with the tube ca. 8 mm long. The calyx limb of these plants often changes form as it ages, with these calyx limbs intiailly tubular and rather shortly lobed then splitting regularly to irregularly, soemtimes to the base. The corolla tubes vary from 5 to 7 mm long in this region. Simira paraensis differs from Simira rubescence basically only by its leaves that are densely hirtellous below; this species is not evaluated here but probably represents a regional form of Simira rubescens rather than a distinct taxon.
Taylor (in Taylor et al., 2004) included Simira erythroxylon var. saxicola in the circumsription of Simira rubescens, but more material now shows these are distinct species and that variety differs in its corollas with the lobes about as long as the tube, acute, and with a thickened projection at the tip and leaves withour domatia that are softly pilosulous through on the lower surface.
Ironically, two separate species of this genus have been named with the epithet "tinctoria". One is Simira tinctoria, the type of the genus and so far only known from French Guiana, and the other is a species long treated in Sickingia. This second name applies to the plants now treated as Simira rubescens, but these two species have been confused in herbarium identifications due to the synonymization of Sickingia with Simira. The name Simira cordifolia has been used for Simira rubescens in the western Amazon basin, at least in herbarium identifications, but that species is restricted to the Magdalena drainage and coast of northern Colombia. Simira rubescens has also been confused in the western Amazon basin with Simira wurdackii. The name Sickingia xanthostema has been synonymized with Simira rubescens (Steyermark, 1972; Barbosa & Peixoto, 1989), but this does not seem to match and is provisionally separated here; its identity is unclear, though.