The Santarem marmoset is a small monkey, with a head-body length of 20 to 27 cm (7.9 to 10.6 in). Adults have been reported to weigh anything from 280 to 475 g (9.9 to 16.8 oz). 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.