The California red-sided garter snake is a subspecies of the common garter snake. This slender subspecies of natricine snake is indigenous to North America and is one of three recognized subspecies of Thamnophis sirtalis found in California. While commonly confused with the subspecies T. s. concinnus, it is biologically part of the population of the subspecies T. s. tetrataenia, as pointed out by Boundy and Rossman (1995), but was preserved as T. s. infernalis as a neotype under ICZN code Article 75 in a 2000 decision by the International Commission on Zoological Nomenclature (ICZN) in 2000 in order to preserve the existing subspecies taxonomy.

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.