BACKGROUND: Changes in body composition during cancer treatments have been linked with poorer outcomes, and increased morbidity and mortality. The effect of resistance training (RT) on body composition in cancer cohorts is debated. OBJECTIVE: We conducted a systematic review and meta-analysis to determine the effect of RT on body composition during and after treatment. METHODS: We searched five electronic databases for articles up to 1 February 2021 and included randomized controlled trials that compared RT with a non-exercise control in adults with cancer. Risk of bias was assessed using the RoB 2 tool. Pairwise, random-effects meta-analysis was used to synthesize the available data. RESULTS: Overall, we included 15 studies (n = 1368). After treatment (11 studies), RT increased lean mass with moderate heterogeneity {0.41 kg [95% confidence interval (CI) 0.05, 0.76], p = 0.029; I(2) = 47.1%, p = 0.02} and decreased fat mass with substantial heterogeneity (- 0.59 kg [95% CI - 1.05, - 0.12], p = 0.019; I(2) = 69.1%, p < 0.001). During treatment (4 studies), RT did not increase lean mass (0.71 kg [95% CI - 0.04, 1.45], p = 0.05; I(2) = 0.0%, p = 0.75) or reduce fat mass (0.00 kg [95% CI - 5.31, 5.30], p = 0.99; I(2) = 0.0%, p = 0.62), both with no heterogeneity. CONCLUSION: Modest improvements in body composition were observed following RT after cancer treatment; however, no changes were observed during treatment. These adaptations are markedly lower than those observed in healthy cohorts but may be clinically meaningful for the cancer survivorship population. At present it is unclear if these diminished adaptations are due to ineffective exercise prescriptions in cancer cohorts or due to an innate anabolic resistance as a result of cancer and its treatments. STUDY REGISTRATION: Open Science Framework (osf.io/x6z72).