Weedy rice, a pervasive and troublesome weed found across the globe, has often evolved through feralization of rice cultivars. In the South American Southern Cone region, rice breeding has a contemporary history and weedy rice has been reported as an important constraint since early 1970s; however the origin and genetic composition of Argentinian weedy rice has not been explored so far. To study this, we used genotyping-by-sequencing to generate genome-wide single nucleotide polymorphisms (SNPs) and compared Argentinian weedy rice with whole-genome data from strains, rice cultivars and wild rice accessions from regions worldwide. In addition, we conducted a comprehensive seed and phenological characterization, and performed an herbicide resistance screening, taking into account mutations in the ALS gene observed in the Argentinian weedy rice collection. Our results revealed large variability in seed traits and flowering time in Argentinian weedy rice. Additionally, most strains were resistant to ALS-inhibiting herbicides with a high frequency of the ALS mutation (A122T), which is also present in Argentinian rice cultivars. Cultivated rice varieties in Argentina belonged to the three major groups: japonica, indica and aus. In contrast, Argentinian weedy rice strains were mostly aus and aus-indica hybrids, resembling cultivars and weedy rice strains from the Southern Cone region. Our findings suggest that Argentinian weedy rice descends from aus weeds, presumably from the United States, which contemporaneously hybridized with local indica cultivars. The observed genetic similarities within the Southern Cone region can be attributed to the continuous exchange of breeding germplasm and cultivars, which, in turn, facilitated the dissemination of weedy rice strains.