Aristolochia cochinchinensis Do (Aristolochiaceae), a new species from southern Vietnam, is described and illustrated. It is characterized by a 6–8 cm long petiole with visible, cylindrical leaf scars, 5–9-flowered cymose inflorescences in clusters of two to four cymes, and a sessile utricle. Information about the distribution, habitat, and phenology as well as a comparison with four morphologically similar species (A. longeracemosa, A. poomae, A. pothieri and A. yalaensis) is provided. Synapomorphic characters such as an abaxially concave perianth and a completely monosymmetric floral limb with a complete fusion of the three sepals into one or two lobes places A. cochinchinensis in subgenus Aristolochia.