Capital breeding marine mammals such as northern elephant seals are uniquely adapted to tolerate prolonged fasting periods associated with terrestrial reproduction and molting. We used a proteomics approach to gain insights into cellular mechanisms of energy provisioning during fasting in elephant seals. We collected blood, blubber, and muscle from adult female elephant seals at the beginning and end of a month-long molting fast and compared protein expression between samples using label-free protein quantification by LC-MS/MS. We identified 309 proteins in plasma, 743 proteins in muscle, and 1098 proteins in blubber. Fifty proteins were differentially expressed in plasma, while only two proteins were differentially expressed in both muscle and blubber between early and late fasting stages. Apolipoprotein A-IV was significantly downregulated in all three tissues in late compared to early fasting. Proteins upregulated over fasting in plasma included corticosteroid-binding globulin, zinc-alpha-2-glycoprotein, and angiotensinogen, while ferritin was the only protein upregulated in muscle. Proteins downregulated in plasma included alpha-2-macroglobulin and apolipoprotein E, which was also downregulated in blubber. Changes in protein expression were consistent with high levels of lipolysis and endogenous glucose production, insulin resistance, and cortisol elevation observed in fasting seals. These data suggest that the proteomes of energy-utilizing and energy-provisioning tissues remain remarkably stable during periods of high energy expenditure coupled with food deprivation in a fasting-adapted mammal.