The natural range extension and colonization of a new continent by a bird species is rare, and even more rarely is it observed and documented. In 1980, six pairs of Barn Swallows were found breeding in Argentina within the species' historical wintering range, and this South American population has since grown to thousands of pairs. We explored the genetic context and consequences of this natural trans-hemispheric colonization event via comparisons among the South American population and two North American populations. We tested for evidence of a genetic founder event by assessing allelic diversity at eight microsatellite loci and haplotype diversity of mitochondrial ND2 sequences. Contrary to our expectations, the recently established South American breeding population showed no evidence of a founder effect, with no difference in heterozygosity, allelic diversity, haplotype diversity, or population differentiation in comparison to the large North American populations. The genetic similarity of these populations suggests that this long-distance colonization event was not associated with a strong demographic bottleneck, perhaps because the South American population has been augmented by ongoing immigration from North America.