A three-dimensional geometric morphometric approach was employed to examine shape variation in laser-scan generated models of lower second molars and its relationship to diet in a sample of 9 extant and 16 extinct platyrrhine genera. Principal component analysis of twenty-three x,y,z landmarks describing the occlusal table and sidewalls showed that dental relief was the main contributing factor to variation along the first axis. Discriminant function analysis (DFA) of PC 1 scores and centroid size accurately classified extant platyrrhines according to dietary preference; however, without centroid size, the DFA was less successful. Within this framework, most of the fossil platyrrhines, including specimens from Patagonia, Colombia, Brazil, and the Caribbean, were predicted to have had a frugivorous diet, but several taxa were classified as having a frugivorous/insectivorous diet, the middle Miocene Neosaimiri, Patasola, and Laventiana, all from La Venta. Alouattins, including the La Ventan Stirtonia and the Cuban Paralouatta, showed variable classification as either frugivores or folivore/frugivores. Xenothrix, from Jamaica, was classified either as a frugivore or frugivore/omnivore. Dietary profiles across different extinct platyrrhine communities are compared and discussed in a paleoecological context. Anat Rec, , 2011. © 2011 Wiley Periodicals, Inc.