Apex predators such as Pacific white-sided dolphins (Lagenorhynchus obliquidens) have the potential to impact prey populations and to be affected by changes in prey abundance. As abundant predators that range widely across the North Pacific Ocean, their interactions with prey populations may have conservation implications. The energy required by individual Pacific white-sided dolphins was estimated using a bioenergetic model that accounted for different age classes and reproductive stages (calf, juvenile, adult, pregnant, and lactating). Monte Carlo simulations incorporating variability in model parameters (i.e., body mass, growth rate, costs of gestation and lactation, metabolic rate, cost of activity, and assimilation efficiencies) were used to predict ranges in energetic requirements of this species. Mean (± SD) total energy requirements in MJ/day were 40.3 ± 6.2 for calves, 70.8 ± 8.2 for juveniles, 69.0 ± 3.6 for adults, 70.3 ± 3.6 for pregnant females, and 98.4 ± 20.0 for lactating females. Estimates of energy requirements were most sensitive to uncertainty in values used for resting metabolic rates and energetic costs of activity. Estimated mass-specific energy requirements in MJ kg⁻¹ day⁻¹ were elevated in calves (1.55 ± 0.23), juveniles (0.97 ± 0.11), and lactating females (1.01 ± 0.21) when compared with nonreproductive adults and pregnant females (∼0.71 ± 0.04). Based on a high-energy diet, an average-sized dolphin (78 kg) would consume approximately 12.5–15.8 kg of fish or 16–20% of its body mass per day. These high energetic requirements may indicate a reliance of dolphins on energy-rich prey, which has implications for fisheries management and conservation of marine mammals.