The Northwestern Garter Snake is the smallest garter snake in British Columbia and can range in size from 355 to 750 mm in snout vent length, although they are usually less than 600 mm (Matsuda et al. 2006). Males are smaller in head dimensions and body size than females, although they have longer tails relative to body length.
Northwestern Garter Snakes exhibit great colour variation; however, most are non-shiny brown to grey in colour, often with a slight checkering on the dorsal surface. Some individuals are melanistic and are uniformly black, and albino individuals (all white/cream in colour) have been found. Dorsal and lateral body stripes range in colour from pale yellow to bright orange, and may be partial or indistinct in many individuals. Ventral scutes range in colour from white to light yellow to black and sometimes have red to orange blotches on the surface. The head is small and not distinct from the neck, eyes are small with round pupils, and the snout is often referred to as blunt. There are usually 7 white upper labials, often with red to black blotch markings. Dorsal body scales are lightly keeled and have up to 17 scale rows at mid-body. Snakes have a single anal plate scale and paired ventral scales posterior of the cloaca. When a snake is nearing ecdysis (shedding their skin) the body color will be dull and eyes will appear blue-grey in colour as fluid lymph fills the area between the new and old layers of skin.