Platarctia parthenos (St. Lawrence tiger moth) is a moth in the family Erebidae. It was described by Harris in 1850. It is found in boreal North America, ranging from Alaska to Labrador, south to New Mexico and Arizona in the Rocky Mountains and to North Carolina in the Appalachian Mountains. The habitat consists of riparian areas and mixed hardwood-conifer forests at middle to high elevations. The larvae feed on various plants, including Salix, Alnus and Betula species. The species first overwinters as a fifth instar larva and again as an eighth instar larva.