BACKGROUND Chemotherapy is a primary adjuvant treatment strategy for breast cancer patients, accompanied by weight gain and negative changes in body composition. However, it is unknown whether exercise is effective in preventing said weight gain and mitigating body composition changes of breast cancer patients undergoing treatment. OBJECTIVE The current study used meta-analysis with trial sequential analysis to determine exercise effect on weight and body composition of breast cancer patients undergoing chemotherapy. METHODS Cochrane Library, PubMed, EMBASE, EBSCO, Scopus, and SinoMed were searched (from the database start date up to August 16, 2021) for randomized controlled studies evaluating the effect of exercise on weight or body composition among breast cancer patients during chemotherapy. RevMan software and TSA Software were used to assess the risk of bias and analyze study results. RESULTS In total, 13 studies comprising 1828 participants were included. Meta-analysis showed that exercise could lead to lower weight, body mass index (BMI), and percentage of body fat during chemotherapy for breast cancer patients, and muscular strength showed significant improvement. Trial sequential analysis showed that evidence of muscular strength was sufficient, but BMI evidence requires further confirmation. CONCLUSIONS This meta-analysis found significant differences in body weight, BMI, percentage of body fat, and muscular strength between exercise intervention groups and control groups. CONCLUSIONS Exercise during chemotherapy is beneficial in preventing weight gain and negative changes in body composition. Medical practitioners should encourage patients to start exercising during chemotherapy. However, further studies are required because insufficient sample sizes meant that outcomes of body composition remain unconfirmed.