Porcellio laevis ( the swift woodlouse, dairy cow isopod, or smooth slater in ) is a  of  in the genus . As the species epithet laevis as well as the vernacular name "swift woodlouse" suggests, the species is capable of quick bursts of speed when provoked.

This species of woodlice is distinctively large, appearing up to 20mm long with a smooth dorsal surface. The males can be identified by their long, spear-shaped . Porcellio laevis has a smooth dorsal surface, which separates its visually from many other species of the Porcellio genus.