The pelage above is rather uniformly grayish or reddish brown, more or less grizzled with darker hairs, with slightly paler coloration below. The coat is composed of sparse guard hairs and thicker underfur. The young are grayer than the adults. The sewellel's size, uniform color, and apparent absence of a tail distinguish it from all other mammals within its range. Females have six mammae.
Males have abdominal testes and a baculum.
The cheek teeth grow throughout life. The skull is flattened and greatly widened posteriorly, and the lower jaw is large and powerful. The sewellel is neither a beaver nor a particularly mountain-loving animal. It is sometimes found at up to 2,200 meters in elevation but is more common at lower levels. It frequents forests and densely vegetated thickets, generally living in areas where there is heavy rainfall or along seepages and streams, where the ground is usually saturated with water.