The Northern water snake can grow up to 135 cm (4.4 ft) long. They can be brown, gray, reddish, or brownish-black. They have dark crossbands on their necks and dark stripes and blotches on the rest of their bodies, often leading to misidentification as cottonmouths or copperheads by novices. They darken as they age. Some will become almost completely black.

Northern water snakes have many predators, including birds, raccoons, opossums, foxes, snapping turtles, and other snakes. They defend themselves vigorously when they are threatened. If they are picked up by an animal, or person, they will bite repeatedly, as well as release excrement and musk. Their saliva contains a mild anticoagulant, which can cause the bite to bleed more but poses little risk to humans.