Garter snake is a common name for 30 species of harmless colubrid snakes of the genus Thamnophis. They are found from the southern Northwest Territories to Costa Rica. Most have a prominent, longitudinal stripe down the back, and on many species, a parallel stripe below on each side, but stripeless, spotted species also occur. Six species have been recorded in Canada, including the common garter snake (T. sirtalis), which has the widest range of any North American snake. Garter snakes are often abundant and occupy a wide variety of terrestrial and semi-aquatic habitats, including urban settings.

Diets vary among species but include slugs, earthworms, fish and amphibians. Mammals are eaten occasionally, especially by the western garter snake (T. elegans), which restrains such prey by a primitive coiling behaviour. All are live-bearing and normally produce 5-25 young by mid- to late summer but much larger litters are reported occasionally.