The Garter snake is a Colubrid snake genus (Thamnophis) common across North America, ranging from Alaska and Canada to Central America. It is the single most widely distributed genus of reptile in North America^{[citation needed]}. The garter snake is also the Massachusetts state reptile.

There is no real consensus on the classification of species of Thamnophis. Disagreement among taxonomists and sources, such as field guides, over whether two types of snakes are separate species or subspecies of the same species is common. They are also closely related to the snakes of the genus Nerodia, and some species have been moved back and forth between genera.

Garter snakes spread throughout North America. The common garter snake (Thamnophis sirtalis), is the only species of snake to be found in Alaska, and is one of the northernmost species of snake in the world, possibly second only to the Crossed Viper, Vipera berus. The genus is so far ranging due to its unparticular diet and adaptability to different biomes and landforms, with varying proximity to water. However, in the western part of North America, these snakes are more aquatic than in the eastern portion. Northern populations hibernate in larger groups than southern ones.