Previous studies evaluating the effects of betaine supplementation on body composition offer contradictory findings. This systematic review and meta-analysis assessed the effects of betaine supplementation on body composition indices (body mass [BM], body mass index [BMI], body fat percentage [BFP], fat mass [FM], fat-free mass [FFM]), and dietary intakes. Studies examining the effects of betaine supplementation on body composition and dietary intakes published up to August 2021 were identified through PubMed, the Cochrane Library, Web of Science, Embase, SCOPUS, and Ovid databases. Betaine supplementation failed to significantly affect BM [(WMD: −0.40 kg, 95% CI: −1.46 to 0.64), p=0.447], BMI [(WMD: −0.05 kg/m2, 95% CI: −0.36 to 0.25), p=0.719], BFP [(WMD: 0.26%, 95% CI: −0.82 to 1.36), p=0.663], FM [(WMD: −0.57 kg, 95% CI: −2.14 to 0.99), p=0.473], and FFM [(WMD: 0.61 kg, 95% CI: −1.27 to 2.49), p=0.527]. Subgroup analyses based on participant’s age (<40 and >40 years), sex, BMI, trial duration (<8 and ≥8 weeks), betaine supplementation dosage (<4 and ≥4 g), and health status (healthy or unhealthy) demonstrated similar results. Other than a potential negligible increase in protein intake (WMD: 3.56 g, 95% CI: 0.24 to 6.88, p=0.035), no changes in dietary intakes were observed following betaine supplementation compared to control. The present systematic review and meta-analysis does not show any beneficial effects of betaine supplementation on body composition indices (BM, BMI, FM, and FFM).