"Santamartamys" specimens measure between from head to the tip of their tail, with their tails measuring between . The species can weigh up to , and has a woolly, soft, and long coat. The species is mainly rufous. The hair on the dorsal region is of intense red colouring, and a large portion of the tail hair is black (basal), but the last two-fifths of the tail are white. Its ears are small and light brown, and feature tufts of long hair on the inner surface, but lack hair on the outer surface. Between the eye and the ear, there is a tuft of long black hairs. The thin whiskers can reach up to long, and has a strip of red fur around its neck.