The gray-blue flat-tailed sea snake has a large head, indistinct head and neck distinction, and a cylindrical body. The back is blue-gray, the ventral surface is gray-yellow, the whole has blue-black rings, the head is blue-black, and the upper lip is gray-yellow from the muzzle to the temple.

The grey-blue flat-tailed sea snake has a body length of 75 to 360 cm, an average of 125 cm, and a weight of 600 to 1800 grams. Females are larger and heavier than males. The gray-blue flat-tailed sea snake has 21 to 25 rows of irregularly distributed scales. The body is mainly of a slightly squashed sub-tubular shape, with a raised muzzle and a large scale on the forehead.