The aim of this study is to assess the effectiveness of swimming training in reducing body mass index (BMI) and body fat percentage among individuals with Down syndrome (DS), utilizing a meta-analysis approach. We conducted a systematic search for studies examining the clinical impact of swimming training on individuals with DS. Inclusion criteria encompassed studies involving DS patients who underwent swimming training for a minimum duration of 4 weeks. The primary focus was to compare changes in BMI and body fat percentage from baseline to post-training. Our analysis incorporated four studies comprising a total of 48 participants. The results demonstrated that swimming training lasting from 8 to 36 weeks did not significantly decrease BMI among DS patients, with a weighted mean difference (WMD) post-training of −0.428 (95% confidence interval (CI): −1.721–0.865) and an I2 of 0%. Subgroup analysis based on age or duration (≥33 weeks or 8 weeks) also showed no notable decrease in BMI. Conversely, the analysis of body fat percentage demonstrated a significant decrease, with a WMD of −2.946 (95% CI: −5.056–−0.837). Subgroup analysis indicated a consistent reduction in body fat percentage across all duration groups. Swimming training does not lead to a significant reduction in BMI among individuals with DS. This outcome may be attributed to concurrent muscle gain alongside decreased body fat, resulting in a notable decrease in body fat percentage.