Aim: To summarize evidence regarding the effects of oral nutritional supplement on muscle fitness of patients undergoing dialysis. Design: A systematic review and meta-analysis. Review sources: Five English databases (CENTRAL, PubMed, EMBASE, CINHAL, and Web of Science) and four Chinese databases (CNKI, WanFang, SinoMed, and VIP) were searched from inception to 31 July 2019 and only randomized controlled trials were included. Review methods: Two reviewers independently searched these databases, selected trials, conducted bias assessment, extracted the data. Random-effects meta-analysis was conducted to assess the effect size. The predetermined subgroup included type of oral nutritional supplement (a mixture of macronutrients, whey protein, essential amino-acids, and other nutrients) and intervention duration (over and less than 48 weeks). The subgroup analyses and sensitivity analyses were conducted to explore source of heterogeneity and robustness of results. Results: Sixteen studies (910 participants) meeting the inclusion criteria were identified and included in this systematic review. Subgroup analysis showed that supplying a mixture of macronutrients (MD [MD] = 2.36 kg, 95% CI [0.45, 4.26], I2 = 0.00%), an intervention duration of 48 weeks (MD = 4.05 kg, 95% CI [1.43, 6.67], I2 = 0.00%) had some effects on increasing lean body mass. No effects of oral nutritional supplement were found on improving muscle strength or physical performance. Conclusion: A mixture of macronutrients and an intervention duration as long as 48 weeks had some significant effects on improving lean body mass of patients undergoing dialysis. No effect of oral nutritional supplement on muscle strength or physical performance were found but with limited evidence. Impact: No existing reviews have ever focused on improving muscle fitness of patients undergoing dialysis. This systematic review and meta-analysis provided evidence of oral nutritional supplement on keeping muscle fitness of these patients and suggested possible type of oral nutritional supplement and intervention duration for clinical practice. Keywords: dialysis; meta-analysis; muscle fitness; muscle mass; muscle strength; nursing; oral nutritional supplement; physical performance; sarcopenia; systematic review.