Cottonmouth Snakes are large, aquatic, venomous snakes. Cottonmouth Snakes are generally dark coloured with tones of olive, brown or black. A lighter/darker cross banding pattern may be seen, especially on their sides. The dorsal banding pattern fades with age, so that older individuals are an almost uniform olive brown, greyish brown or black.

Their belly is white, yellowish white or tan, marked with dark spots and becomes darker near their posterior. The amount of dark pigment on the belly varies from virtually nothing to almost completely black. Their head is more or less uniform brown colour. Cottonmouth Snakes are heavy bodied with keeled scales.

Cottonmouth Snakes can be aggressive. Unlike their Copperhead cousins, they will often stand their ground. Cottonmouth Snakes venom is stronger and they tend to be larger snakes, making them very dangerous.

However, in tests designed to measure the suite of behavioural responses by free-ranging cottonmouths to encounters with humans, 51 percent of the test subjects tried to escape and 78 percent used threat displays or other defensive tactics. Only when the snakes were picked up with a mechanical hand were they likely to bite.