The head and body usually have a greenish dorsal color, shading to yellow-greenish along the sides.

The belly is generally lighter in color: cream, yellow-green or pale green.

The dorsal pattern may have irregular blotches, turquoise to black, or speckling that doesn't reach very far down the sides.

Like all other pitvipers, B. thalassinus has heat sensitive organs, or loreal pits, located on either side of the head between the eye and the nostril.

B. thalassinus is not known to be an aggressive species, but may strike quickly when surprised or disturbed.