Tamarins are small-bodied Neotropical primates that mainly feed on ripe fruits and insects, and supplement their diet with plant exudates, nectar, fungi and vertebrate prey. They are important seed dispersers in Neotropical forests as they are able to disperse a large number of small and medium-sized seeds from parental trees. In this paper, we describe the diet of the white-footed tamarin (Saguinus leucopus) and its role as seed disperser in a fragmented landscape in Colombia. During a twelve month period, we collected data on activity patterns, ranging behavior and feeding ecology, as well as on habitat-wide forest productivity. S. leucopus fed from >95 plant species and spent 17% of their time feeding. We found a positive relationship between fruit consumption and ripe-fruit availability. Dietary diversity increased during periods of fruit scarcity, when the tamarins fed more on insects and exudates. During periods of fruit scarcity, they relied more heavily on the borders of forest fragments and made occasional incursions into adjacent fragments across a matrix of pastures. They used larger areas and had larger overlapping home ranges during periods of fruit scarcity. We recovered at least 44 species of seeds effectively dispersed by tamarins, including small and medium-sized seeds (range <1–26 mm). Seeds were dispersed up to 500m from parent trees across a wide diversity of habitat types. This study provides further evidence on the important role small-bodied frugivorous primates play in the recovery of forest connectivity and in the maintenance of tropical forest diversity in human-impacted landscapes.