This study evaluates the effect of whey protein (WP) supplementation with resistance training (RT) on body mass and muscular strength through randomized controlled trials (RCTs). A literature survey was conducted using electronic databases, and study selection was based on predetermined eligibility criteria. Meta-analyses of mean differences (MD) in lean/fat mass or standardized MD (SMD) in muscular strength between WP-RT and placebo-RT groups were conducted along with sensitivity and subgroup analyses. Metaregression analyses were performed to identify factors affecting the change in lean/fat mass. Data were used from 21 RCTs in which 837 participants attended 13.1 weeks [95% CI 10.5, 15.7] of RT. In comparison with the placebo-RT group, WP-RT exhibited improved lean mass (MD 0.46 kg [−0.02, 0.94]; p = 0.01), fat mass (MD −0.62 kg [−1.05, −0.19]; p = 0.004) and muscular strength (SMD 0.25 [0.11, 0.38]; p = 0.0003) in healthy individuals but not in individuals with a pathological condition. Moreover, in comparison with the placebo-RT group, the WP-RT group showed improved lean mass (MD 0.38 kg [−0.03, 0.79]; p = 0.07), fat mass (MD −0.75 kg [−1.09, −0.41]; p < 0.00001) and muscular strength (SMD 0.30 [0.13, 0.47]; p = 0.005) in younger (<40 years age) individuals only. The change in lean mass was inversely associated with age in the overall population. The RT duration was positively associated with the improvement in lean mass in healthy individuals. The improvement in fat mass was positively associated with the body height of individuals. WP supplementation during RT sessions was found to improve lean mass, fat mass and muscular strength in healthy and younger individuals.