The Santarem marmoset is a small monkey, with a head-body length of . Adults have been reported to weigh anything from . Its physical form is similar to that of other marmosets, with lower canines that barely protrude above the line of the incisors and claws (rather than nails) on all of their digits except for the big toe. It most closely resembles the Maués marmoset in appearance, differing in having paler fur and in the shape of the ear tassels. Males and females are similar in size.