The Western Rattlesnake is the only member of the viper family and the only venomous species found in British Columbia. It is easily distinguished from other snake species in the province by the presence of the rattle (a series of modified, interlocking scales on the end of the tail) and sensory heat pits along the upper lip. However, the rattle is not always present, particular in young animals that may not have grown enough and added enough segments onto the rattle in order to generate the buzzing sound or in an older animal where the segments have been broken or damaged. A series of blotches run down the back, developing into more ring-like patters towards the tail. The blotches take on various shapes, particular towards the tail, but tend to be rather square. The animal also has a flattened, triangular-shaped head with a more distinct neck than the other snakes. In British Columbia, the colour of the animal is usually a mixture of greys, olive greens, and darker shades, with whiter bands separating the blotches. Young animals often show more vibrant, brownier tones. In general, the animal appears bulkier and more robust than the other snakes in the province, although for its size, the musculature is relatively less developed than the other snakes.
This is not a large rattlesnake compared to other North American species. Extremely large males can exceed 110 cm from the tip of the snout to the end of the tail, although individuals like this very rare, possibly because the animals rarely live long enough to attain this size.