Studies of interactions between invasive and native species often focus on impacts on natives. We report potential impacts of a native predator, the Lake Erie Watersnake (Nerodia sipdeon insularum) on an invasive fish, the Round Goby (Apollonia melanostomus). Round Gobies have increased exponentially in the Great Lakes and now constitute >90% of prey consumed by Lake Erie Watersnakes. We investigated the effects this shift may have on round goby populations by estimating total prey consumption by Lake Erie Watersnakes. Digestive rate trials and maximum voluntary prey consumption trials indicate that gastric digestion is rapid (digestion was 90% complete after just 16.4 hours at 30°C and 20.1 hours at 25°C) and voluntary prey consumption is high (from 30.0% of adult female body mass to 117% of neonate body mass in five days). Based on palpation of wild-caught snakes, prey were detected more frequently in adult females than adult males, but no such difference was observed in subadults. The proportion of snakes containing prey varied over time with season-long averages of 11.6% for adult females, 6.9% for adult males, and 22.4% for subadults. Systematic surveys by boat indicate that nearly 90% of foraging occurs <150 m from shore. Projected annual consumption, based on gastric digestion rate, maximum voluntary prey consumption, feeding frequency of free-ranging snakes, and published energetic data and a population size of 12,000 adult watersnakes, ranges from 200,000–3,300,000 Round Gobies (4,455–56,178 kg) per year. Although impressive, this rate of prey consumption is unlikely to have more than local effects on Round Goby populations.