Adult black marsh turtles are usually small to medium-sized, averaging at around in length and rarely exceeding . Some individuals, however, are known to grow to twice this size. The ovoid carapace (the dorsal shell) is widest just behind the middle portion, with a strongly serrated posterior margin. A prominent central ridge (known as keels) also runs through the middle of the carapace. Two lateral keels are present as well, though they are not always visible. The carapace is almost entirely black or dark brown with black radiating patterns.