The marmosets and tamarins, Family Callitrichidae, are Neotropical primates with over 60 species and subspecies that inhabit much of South America. Although callitrichids exhibit a remarkable widespread distribution, attempts to unravel their biogeographic history have been limited by taxonomic confusion and the lack of an appropriate statistical biogeographic framework. Here, we construct a time-calibrated multi-locus phylogeny from GenBank data and the callitrichid literature for 38 taxa. We use this framework to conduct statistical biogeographic analyses of callitrichids using BioGeoBEARS. The DIVAj model is the best supported reconstruction of biogeographic history among our analyses and suggests that the most recent common ancestor to the callitrichids was widespread across forested regions c. 14 Ma. There is also support for multiple colonizations of the Atlantic forest region from the Amazon basin, first by Leontopithecus c. 11 Ma and later by Callithrix c. 5 Ma. Our results show support for a 9 million year old split between a small-bodied group and large-bodied group of tamarins. These phylogenetic data, in concert with the consistent difference in body size between the two groups and geographical patterns (small-bodied tamarins and large-bodied tamarins have an unusually high degree of geographic overlap for congeners) lend support to our suggestion to split Saguinus into two genera, and we propose the use of distinct generic names; Leontocebus and Saguinus, respectively.