Polianthes alboaustralis, a new species endemic to the State of Oaxaca, Mexico, is described and illustrated. This species is morphologically similar to Polianthes nelsonii, which is distributed in Durango and Zacatecas, and P. palustris from Nayarit, but it differs from these by having larger leaves, first bract, inflorescence, anthers, and longer filaments; the perigone lobes are broadly ovate, the anthers are longer, and extrafloral nectaries are usually present. A dichotomous key to the species of Polianthes thus far described for the State of Oaxaca is included.