Sapindus saponaria is a deciduous tree that can grow up to 25 meters tall. Its pale brown bark is fissured and sheds on large scales as the tree matures. It has compound (even-pinnate) leaves that are either alternate or opposite. The blades are oblong-elliptic and slightly falcate, measuring 6–16 cm long and 2–4.5 cm wide, with 3–6 pairs of leaflets. The rachis is winged in young leaves. The leaflets have acuminate apexes and rounded bases. The upper surfaces of the leaves are glabrous, while the lower surfaces range from moderately tomentose to puberulent. The leaves are chartaceous (papery) with entire and slightly undulated margins. The petioles, which are 3–5 cm long, are swollen at the base. Stipules are absent. It produces flowers in terminal, tomentose 10–20 cm long panicles. It is monoecious with unisexual, actinomorphic (radially symmetric) flowers. These flowers have pedicels that are 1–3.5 mm long. The calyx consists of 5 sepals that are broadly imbricate and unequal; the outer sepals are ovate, and the inner ones are suborbicular, all pubescent and minutely ciliate. The corolla comprises five petals, each with a 2-lobed pilose scale at their base, subequal, obovate to suborbicular–obovate, strongly concave, and about 2 mm long, with ciliate edges. It has eight stamens with staminal filaments approximately 1 mm long and pubescent at the base; the anthers are ellipsoid, dithecal, and open via longitudinal slits. The ovary is superior, ovoid, 2–3-celled, and glabrous in pistillate flowers but rudimentary in staminate flowers. It has a terminal style and a 3-lobed stigma. The fruits comprise 1(2–3) fleshy, indehiscent schizocarpic globose mericarps, each about 1.7–2 cm long with a coriaceous pericarp. Each mericarp contains one seed, which is dark reddish brown to black, approximately 10–12 mm long, and has a seed coat that lacks a tuft of hairs at its base; there is no endosperm. It is found in mesic forests between 900-1,370 meters.