Garter snake is a common name for generally harmless, small to medium-sized snakes belonging to the genus Thamnophis in the family Colubridae. Native to North and Central America, species in the genus Thamnophis can be found from the subarctic plains of Canada to Costa Rica.

With about 35 recognized species and subspecies, garter snakes are highly variable in appearance. They generally have large round eyes, round pupils, a slender build, keeled scales, and a pattern of longitudinal stripes that may or may not include spots (although some have no stripes at all). They also vary significantly in total length, from 18 to 51 in (46 to 130 cm).

With no real consensus on the classification of species of Thamnophis, disagreement between taxonomists and sources such as field guides over whether two types of snakes are separate species or subspecies of the same species is common. Garter snakes are closely related to the genus Nerodia (water snakes), with some species having been moved back and forth between genera.