Food habits of the Red-backed Hawk (Buteo polyosoma) were studied in the southeastern coast of Buenos Aires Province, Argentina, during autumn and winter of 2001–03. At the study area, the Red-backed Hawk was a specialist predator. On a numerical basis, 99.5% of its prey were small mammals, and little seasonal and annual variability in diet was observed. The diet was composed of murids—mainly field mice (Akodon azarae), pampas rice rats (Oligoryzomys flavescens), and vesper mice (Calomys spp.), and by octodontids (tuco-tucos [Ctenomys talarum]). The tuco-tuco was the most important prey, representing 55% of the diet biomass. The Red-backed Hawk exhibited a sit-and-wait strategy, indicated by the high percentage of time study birds were perched (78.8%). Feeding, comfort movements, and flight were also recorded (11.6%, 4.9%, and 4.7%, respectively). We observed 61 capture attempts on tuco-tucos, of which 42.6% were successful. Hawks displayed agonistic interactions with other bird species. The main aggressor (77.5%, N  =  372) was the Chimango Caracara (Milvago chimango) that frequently consumed prey remains discarded by the hawks. Our work suggests a trend of preying on relatively more small mammals toward the southern end of its geographic range. The Red-backed Hawk in its wintering range seems to be a relative specialist predator mostly employing passive search as its main hunting strategy.