We describe and illustrate Aristolochia nuichuaensis (Aristolochiaceae) as a new species from southern Vietnam. It has a cordate leaf blade, an up to 16(20)-cm-long inflorescence axis, a strongly constricted, oblong cylindrically-shaped upper part of tube, with outer surface uniformly purple, inner surface uniformly white, a campanulate limb, 4.5–5.0 cm in diam., with non-revolute lobes, a whitish outer surface of limb with visible purple streaks, a white inner surface of limb with sunken dark-purple streaks and verrucoses, an elliptic and white throat, and a gynostemium with three acute lobes. Its phenology, ecology, and comparisons with some morphologically similar species are presented. The diagnostic characters place A. nuichuaensis in the subgenus Siphisia. We also provide an identification key and a checklist with information on the distributions of the known Siphisia species in Vietnam.