Avian influenza viruses (AIVs) circulate widely among migratory birds, where frequent reassortment drives the emergence of novel genomic constellations. However, the evolutionary pathways underlying these events remain poorly understood in South America due to historically limited genomic surveillance. Here, we report the detection and evolutionary reconstruction of a low pathogenic avian influenza virus (LPAI) subtype H1N8 isolated from a White-rumped Sandpiper (Calidris fuscicollis) sampled at Lagoa do Peixe National Park (PNLP), southern Brazil, an important migratory stopover along the Atlantic Flyway. Between 2023 and 2025, 1787 samples were collected from wild birds, resulting in six AIV-positive detections during April 2024, from which one H1N8 isolate was successfully recovered and whole-genome sequenced. Genomic analyses identified a reassortant virus sharing five internal gene segments (PB2, PB1, PA, NP, and MP) with a previously described H11N2 virus detected at the same site and host species, whereas the HA, NA, and NS segments belonged to distinct South American lineages. Bayesian temporal reconstruction, discrete phylogeographic analyses, ancestral-state reconstruction, and reassortment inference consistently supported the persistence of a shared North American-derived genomic backbone followed by the acquisition of locally circulating South American gene segments. Although based on a single isolate, these findings provide an evolutionary snapshot of a reassortment event in South American migratory birds and reinforce PNLP as a key site for long-term avian influenza virus surveillance.