Cuniculus paca fur is coarse and there is no underfur. The upper body is dark brown or black and usually has 4 longitudinal rows of white spots on the sides. The belly is white. The forefeet have 4 digits and the hind feet 5 digits. The zygomatic arch is expanded laterally and dorsally and is used as a resonating chamber. This is a unique feature among mammals.

Range mass: 4.000 to 12.000 kg.