Whey protein has several biochemical characteristics which make it an ideal nutritional supplement in cancer. This meta-analysis aims to evaluate the effects of whey on perioperative outcomes in cancer. A systematic review was conducted as per the Preferred Reporting of Systematic Reviews and Meta-analysis (PRISMA) guidelines. The primary outcome was postoperative complications. Secondary outcomes included 6-minute walk test, length of stay (LOS), and thirty-day readmission. Of the ten trials, six supplemented whey to meet protein requirements of around 1.2 mg/kg/day, and four supplemented whey variably. A synthesis of ten trials with 643 patients showed significantly decreased postoperative complications in the whey supplemented (22%) group as compared to the control (32%) (OR 0.61, 95% CI 0.41-0.90; P = 0.01). Analysis of six trials showed that patients supplemented with whey had greater functional walking capacity before surgery (MD 23.76 meters, 95% CI 4.05-43.47; participants = 377; P = 0.02) and after 4 weeks of surgery (MD 45.76, 95% CI 14.19-77.33; participants = 366; P = 0.004). Thirty-day readmissions and LOS showed no differences. Risk of bias varied between the trials and evidence was moderate to low. Whey protein supplementation improved the perioperative functional capacity and significantly reduced postoperative complications in patients with cancer.