Omega-3 fatty acids are bioactive nutrients with the potential to preserve lean body mass in individuals with cancer. This study aimed to review the literature on randomized clinical trials that evaluated the effects of omega-3 supplementation on lean body mass in cancer patients. As secondary objectives, we evaluated the effects of omega-3 supplementation on body mass index (BMI) and body weight. We conducted a systematic review and meta-analysis in the following databases: Pubmed, LILACS, Scielo, Scopus, Web of Science, Cochrane, and Embase. It included randomized clinical trials that investigated the effects of omega-3 supplementation on lean body mass in cancer patients. Observational studies, animal experiments, studies carried out with healthy humans, and non-randomized clinical trials were excluded. We utilized the Cochrane scale to assess the quality of the studies. A meta-analysis was carried out to evaluate the effect of omega-3 on lean body mass, BMI, and body weight. Fourteen studies were included, of which four showed significant results from omega-3 supplementation for lean body mass. In the meta-analysis, omega-3 fatty acids increased lean body mass by 0.17 kg compared to placebo, but without significant differences between the groups [SMD: 0.17; CI 95%: -0.01, 0.35; I2 = 41%]. For body weight, omega-3 showed a statistically significant effect [SMD: 0.26; CI 95%: 0.06, 0.45; I2 = 46%], whereas for BMI the results were not significant. This systematic review and meta-analysis showed no statistically significant effect from omega-3 on lean body mass and BMI. On the other hand, there was a statistical significance for body weight.