Body length: ~1.5 meters Body weight: up to 6.7 kg. Crotalus atrox has a plump body, short tail, and a broad triangular head. Like all Pit Vipers, it has a pit organ, which is situated in an indentation of the upper jaw, between the nostril and eye. The pit is about 5 mm deep, with an outer and inner chamber separated by a thin membrane. The membrane senses very slight temperature differences between the snake's inner and ambient temperatures. The Western Diamond Rattlesnake also has the self-named rattle on the end of the tail. Each link of the rattle is the remnant of a molted skin; as the snake molts, the last scale loosens but does not fall off. As the snake ages, new rattles are formed with each molt, while old rattles simultaneously fall off. This species has long, tubular fangs, which are characteristic of venomous snakes.

Crotalus atrox can be a yellowish gray, pale blue, or pinkish ground color. The diamond shapes down its length are dark with pale white borders. The tail is white with jet-black rings. The head markings include a pale oblique band from nostril to upper labials, and a similar but narrower band behind the eye.