Isodendrion longifolium is a few-branched, virgate shrub that typically reaches heights between 60 cm and 3.6 meters, although it can sometimes grow up to 3.6 meters tall. The branches are either erect or strongly ascending and are leafy up to 20 cm below the apex. The stems are glabrous, contributing to the plant’s streamlined appearance. The leaves are simple and alternate with oblanceolate blades ranging from 10 to 30 cm long and 3.4 to 6.5 cm wide. The leaf apex may be bluntly short-acuminate, acuminate, or obtuse to rounded, while the base is attenuate. The leaf surfaces are glabrous and subcoriaceous, with flat or slightly undulate margins that are remotely serrulate-crenulate, subentire, or rarely bluntly serrate. The veins are conspicuously reticulate and raised on both surfaces, adding texture to the leaves. The petioles are relatively short, between 0.3 and 1.7 cm, and glabrous. Stipules are very slightly asymmetrical and deltate, measuring 2.5 to 4 mm, with a raised midrib and glabrous surface, becoming pale yellow or yellowish-brown as they age. The flowers are solitary and axillary, borne on short pedicels between 2.5 to 4 mm long. The peduncles are minimal to non-existent, and the plant is noted for its fragrant flowers. Each flower consists of a calyx with five ovate to narrowly ovate sepals, a corolla with five purple petals, and five stamens with very short filaments. The superior ovary is 3-carpellate and 1-celled, with parietal placentation. The style is filiform and sigmoid, with an enlarged, excavated apex. The fruit is a purplish, ovoid-ellipsoid capsule approximately 10 mm long, slightly 3-lobed, and tipped with a persistent style. It contains seeds with fleshy, oily endosperm. It is found on steep slopes and cliffs within diverse mesic forests and occasionally wet forests at elevations ranging from 240-700 meters.