The Northern River Otter has a streamlined, muscular body with short legs, webbed toes and a long muscular tail. The North American river otter’s body measure is somewhere between 25.98" to 42.13", and its tail measure is between 12.40" to 18.11";a river otter’s tail makes up 30 to 40% of the total length of its body. It can weigh between 6 and 31 pounds. The river otter has a round, small head, short yet powerful legs, and large whiskers. Otters display sexual dimorphism, as the male otter is often larger than the female. Its fur is glossy and dark brown , and the throat is often silver grey. The otter is a powerful swimmer, but can also travel quickly on land and often propel itself into a rapid slide on its belly on snow or ice; it also likes to slide down river banks into the water. The North American River Otter has nostrils which close underwater and its fur is soft and dense; both of these adaptations help it to have extended excursions underwater.

On land, a Northern River Otter can run up to 18 miles per hour. Its life span is 10-15 years in the wild, but it may live up to 25 years in captivity.