The sewage snail Physa acuta, native to North America, is an effective invasive species around the world. In Chile, it was first reported in 2014 in the north central area of the country. So far, the species has not been recorded in southern Chile. Sampling performed in 2015 in three localities from Llanquihue Lake, Chilean Patagonia, only provided native freshwater snails. However, new collections performed in February 2018 in the same three sites were successful for physid specimens suggesting a biological invasion entailing a large southward range expansion of these snails. Here we performed morphological, microstructural and phylogenetic analyses to investigate whether the new samples belong to Physa acuta. The shell morphology, male copulatory complex and radula microstructure of the new material agree with those of the sewage snail. The molecular phylogenetic analyses using the cytochrome c oxidase subunit 1 (COI) gene confirmed morphological identification. We suggest to take prompt measures to prevent the expansion of Physa acuta in Llanquihue Lake or nearby aquatic ecosystems.