In this study we describe the feeding biology of the exotic predatory whelk Rapana venosa from the Río de la Plata when offered representative coastal bivalves from both soft and hard bottoms. Furthermore, we estimate daily consumption rates using two mussel species commonly found on local rocky outcrops and artificial reefs. R. venosa gastropods (n = 21; shell height (SH), 45–117 mm) were obtained by trawling in the Río de la Plata, and were kept in controlled laboratory settings. To describe its feeding behavior, 10 potential prey species were offered to the gastropods, including Mactra isabelleana, Erodona mactroides, Pitar rostratus, Donax hanleyanus, Mesodesma mactroides, Mytilus platensis, Brachidontes rodriguezi, Mytella charruana, Amiantis purpurata, and Crassostrea gigas. All prey species were consumed. The empty shells left after consumption frequently showed broken shells and rasp signatures on the edges, suggesting a prey attack by marginal chipping or a similar mechanism. This common pattern was not observed in A. purpurata or C. gigas. To explore potential effects of size on feeding behavior, the gastropods were separated into three size classes and offered M. platensis, B. rodriguezi, and M. charruana mussels. Small, intermediate, and large gastropods (SH <60 mm, 60–90 mm, and >90 mm, respectively) consumed comparable mussel size ranges. Small gastropods fed on prey sizes up to 30% larger than their size, whereas large gastropods consumed the largest mussel offered (SH, 78 mm). To estimate daily consumption rates, the 21 gastropods were fed with M. platensis and B. rodriguezi in individual tanks for 12 days. They ate 1.45 ± 0.67 SD mussels and 0.68 ± 0.65 g/day per gastropod. The proportion of tissue wet weight consumed did not change with prey species, but varied among the size classes of R. venosa. Small (<60 mm) and intermediate (60–90 mm) gastropods consumed a similar proportion of prey tissue (up to 12% of their tissue wet weight). In contrast, large gastropods (>90 mm) ate up to 5% of their tissue wet weight. This work is the first report of R. venosa feeding behavior and daily consumption rates in Argentina. The gastropods succeeded in attacking and feeding on a broad variety of local bivalve species characterized by noticeable differences in shell shape, size, and life habits.