The giant pit viper is 60-70 cm long, with a slightly long triangular head, gray-brown to brown back, and a dark ∧-shaped spot on the back of the head, mixed with black spots. There are large symmetrical scales on the back of the head, cheek sockets in front of the eyes, and two longitudinal round spots on the back of the body.

The body color of the giant pit viper changes greatly. There are black bands from the back of the eyes to the corners of the mouth, and there is a yellow-white fine line on the upper edge; the back of the body is alternately arranged with black-brown round spots; the ventral surface is gray-white to gray-black brown, with irregular black spots; the tip of the tail is black.