Black swallow-wort is an herbaceous, perennial, twining vine with single or multiple stems growing from an extensive fibrous root system. Stems are erect initially, but as plants mature they twine around adjacent vegetation or each other for support, often forming impenetrable thickets. Vines are typically 2–6½ ft (60–200 cm) long. Stems are green and darken with age. Leaves are opposite and elliptical with a pointed tip and a rounded or heart-shaped base. Small, star-shaped flowers are produced in clusters at leaf axils. Each flower has five fleshy, purplish-black petals with white hairs. Fruits are thin pods that often occur in pairs and contain numerous tufted, wind-borne seeds.