Western Terrestrial Garter Snakes are medium-sized, diurnal snakes. Due to the wide colour variation exhibited by this species, Western Terrestrial Garter Snakes are the most difficult garter snake to identify and are frequently mixed up with Northwestern and Common Garter Snakes, as well as smaller Gopher Snakes in the Okanagan. Adults have a large heads relative to neck size, and medium to large eyes with round pupils. Lips have 10 lower and 8 upper labials, often with vertical black markings on the front edges of the scales. Dorsal body scales are strongly keeled and have up to 21 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. In British Columbia, Western Terrestrial Garter Snakes can range in size from 150 to 750 mm in snout-vent length (Waye 1999; Isaac 2010). Garter snakes exhibit sexual size dimorphism; males are smaller in head dimensions and body size than females, although they have longer tails relative to body length; such sexual dimorphism has been attributed to reproductive investment in females, as fecundity increased with body size.
There are two distinct colour morph variations of this species in British Columbia: a darker coastal type and a lighter interior type. Both morphs typically have three stripes running the length of their bodies (one dorsal and two lateral); however, there is noticeable variation in colouration, length and visibility of each stripe between individuals and populations (Isaac 2010). The interior colour morph, which is the predominant form, is lighter in dorsal body colour (dusty brown to beige) and usually has a wavy yellow dorsal line, lined by two rows of dark spots. Lateral stripes, located on the second and third scale rows on each side of the body, are yellow and often lighter and less distinct that the dorsal strip. The darker coastal morph typically has a dark body (similar to Common Garter Snake colouration), with a wavy or straight orange-to-yellow dorsal strip and two yellow lateral stripes. The ventral scutes can range in colour from light blue/grey to dark black, and may have irregular markings. When a snake is nearing ecdysis (shedding their skin) the body color will be dull and eyes will appear bluish in colour as fluid lymph fills the area between the new and old layers of skin.