Emerging viruses represent a continuous threat to human health and to farmed animals, as evidenced on multiple occasions by outbreaks of influenza, henipavirus and SARS. Knowledge about the diversity of viromes present in reservoir species can lead to a better understanding of the origin of emerging pathogens. In this study, we extend the knowledge of astrovirus diversity in pigs by reporting the genetic characterization of an unknown astrovirus lineage. Phylogenetic analyses provided evidence that this porcine astrovirus lineage is unique and does not appear to share a recent common ancestor with any known mamastrovirus. The data reported in this study extend the number of porcine astrovirus lineages to a total of five, all of which most likely represent distinct species of different origins.