We studied the diet of the Southern Caracara (Caracara plancus) at Mar Chiquita Biosphere Reserve, Buenos Aires province, Argentina. We identified 3033 prey items (two invertebrate and five vertebrate classes) corresponding to 116 prey types, from 158 prey remains and 558 pellets collected from five pairs of caracaras. Overall food-niche breadth at the species level was 11.5 (N  =  110 species), and the majority of prey items were insects (69.5%), followed by mammals (23.9%) and birds (5%). This pattern was noted in spring and summer; in autumn and winter, mammals dominated the diet sample and insects were recorded less frequently. In all seasons, mammals constituted the majority of prey biomass, followed by birds; insects represented only a small proportion of total prey biomass. Items suspected to be carrion constituted a larger proportion of the diet in winter and included primarily mammals and birds. Diet breadth was highest in winter, and dietary similarity was greater between spring and summer and between autumn and winter. Our results differ from those reported for the diet of Southern Caracaras in northwestern Patagonia probably due to differences in carrion availability between the two regions.