The freshwater lake deposits from the Eocene Green River Formation are some of the most productive fossil fish sites in the world, producing millions of specimens. Nearly thirty species of fish have been identified from the deposits, some represented by thousands of individual specimens. This incredible abundance of specimens provides a great opportunity to investigate aspects of ecology in a fossil community. There is much interest in reconstructing ancient food webs, but understanding these connections between species requires knowledge of the diets of the fish within the communities. Diet inferences are often based on skull and jaw morphology due to the absence of direct evidence of feeding (i.e. stomach contents or preserved feeding behavior). However, the lack of a quantitative framework for estimating diet based on the morphology of extant fish means that these inferences are usually made on a qualitative basis. Here I estimate the diets of fish from the Green River Formation using a framework based on extant actinopterygians. I measured 11 characteristics that are relevant to jaw and skull function during feeding from museum specimens of 27 species of extant actinopterygians. Broad diet groups were determined for these species by using cluster analysis to group species based on the similarity of the proportions of different categories of food items in their diets. I performed a linear discriminant analysis (LDA) to determine which combinations of jaw and skull measurements were best at discriminating among the diet groups. The same 11 jaw and skull characteristics were measured from 6 species of Green River fish, and these measurements were inputted into the LDA trained by the extant species. Diplomystus dentatus, Phareodus encaustus, and Astephus antiquus are reconstructed as fish eaters, Knightia alta and Phareodus testis are reconstructed as invertivores, and Mioplosus labracoides is reconstructed as an arthropodivore. These results are consistent with preserved feeding behavior, such as fish found in the mouths of large Diplomystus and Phareodus individuals, and the feeding habits of extant relatives. Even in the absence of direct evidence of feeding, morphology can be used to reliably estimate the diets of extinct fish.