Adult nutria are about 14 inches long from the nose to the base of the tail. The tail itself is 12 to 17 inches long, round, and hairless. Coloration is brownish, and both sexes are similar in appearance and weight. The nutria is unique in that it has 3 sets or lengths of fur. Primary guard hairs are about 3 inches in length. Beneath this layer is the secondary guard hairs, which are more numerous and give the species its overall coloration. The underfur is short, and less dense than either a muskrat or beaver underfur.

The whiskers on a nutria are obvious. These whiskers are about 4 inches in length, and very numerous. Teeth number 20, and include 4 large incisors, and allows the nutria to cut off underwater plants without getting water into its mouth. The mouth also has glands located near the corners which produce oils that the nutria uses to comb and waterproof its fur. Nutria average 16 to 18 pounds in weight. Occasional individuals may weigh 25 pounds or more. Also unique is the location of the mammary glands on the females. The teats are locate high on the sides of the nutria, which allows young nutria to nurse as the mother swims in the water.

Front feet have five toes, including a small toe corresponding to our thumb. Hind feet are much larger, and unique in that all toes are connected by a skin web except for the toe corresponding to our little toe.