The common garter snake is a species of snake in the subfamily Natricinae of the family Colubridae. The species is indigenous to North America and found widely across the continent. There are several recognized subspecies. Most common garter snakes have a pattern of yellow stripes on a black, brown or green background, and their average total length is about 55 cm (22 in), with a maximum total length of about 137 cm (54 in). The average body mass is 150 g (5.3 oz). The common garter snake is the state reptile of Massachusetts.

The Natricinae are a subfamily of colubroid snakes, sometimes referred to as a family (Natricidae). The subfamily comprises 36 genera. Members include many very common snake species, such as the European grass snakes, and the North American water snakes and garter snakes. Some Old World members of the subfamily are known as keelbacks, because their dorsal scales exhibit strong keeling.