Apantesis williamsii, or Williams' tiger moth, is a moth of the family Erebidae. It was described by Charles R. Dodge in 1871. It is found in North America from the Northwest Territories east to the northern Great Lakes region, New Brunswick and New England. It also occurs throughout the northern Great Plains, south at higher elevations to Arizona and New Mexico, west to south-eastern British Columbia and eastern California. Larvae have been reared on a variety of forbs, and is probably a general feeder on herbaceous plants and grasses.