A high genetic variability has been recognized in Puccinia sorghi (causal agent of common rust of maize) in Argentina, although its origin remains unclear since the different reported alternate hosts (Oxalis corniculata, O. stricta, O. bowiei) have never been detected with this disease in the region. In the spring of 2013 and 2014, spermagonium and aecial estages of a Puccina sp. were observed on O. conorrhiza (syn. O. cordobensis) in Córdoba Province, in central Argentina. Those structures were found in 22 sampling sites, under natural infections, in a radius of 175 km of Córdoba City. On approximately one-third of the leaves of each infected plant, ampulliform, subepidermal, amphigenous spermagonia, arranged in small clusters of 0.5 mm, were observed. Spermagonia containing spermatia and receptive hyphae were golden yellow to orange yellow with abundant nectar exuding. Those in the centre of the lesion are surrounded by annular groups of aecia, formed exclusively on the abaxial surface of the leaves. Aecia were orange, cylindrical short, with irregular opening at the apex. Based on morphological characteristics, DNA sequencing and pathogenicity test, the causal agent was identified as P. sorghi. This is thought to be the first report of P. sorghi isolated from O. conorrhiza worldwide.