INTRODUCTION: Physical capacity and quality of life (QoL) are typically impaired in children/adolescents with cancer. Our primary objective was to examine the effects of exercise training performed after diagnosis of any type of pediatric cancer on physical capacity-related endpoints, survival, disease relapse and adverse effects. METHODS: (a) Search and selection criteria: Systematic review in Pubmed and Web of Science (until August 2018) of randomized controlled trials (RCTs) of exercise interventions in children with cancer during treatment or within one year after its end. (b) Data collection: Two authors independently identified studies meeting inclusion criteria, extracted data, and assessed risk of bias using standardized forms. When needed, we contacted authors to request clarifications or additional data. (c) Statistical Analysis: The pooled standardized mean differences (SMD) were calculated for those endpoints for which a minimum of three RCTs used the same assessment method. We also calculated the pooled odds ratio (OR) of dying or having a tumor relapse. RESULTS: Eight RCTs (n = 283 patients) were included in the systematic review. Of these, five studies (n = 173) could be meta-analyzed. Exercise training during pediatric cancer treatment significantly improves patients' functional mobility, as assessed with the timed up and down stairs test (SMD: -0.73, p < 0.001) and does not affect mortality (OR: 0.85, p = 0.76) or relapse risk (OR: 0.96, p = 0.94). CONCLUSIONS: Exercise training in children with cancer improves functional mobility. More RCTs analyzing the effects of supervised exercise interventions are needed, as well as the development of a core-set of outcomes in pediatric oncology exercise research.