Background: Overweight and obesity is a growing concern among individuals with intellectual disabilities; however, little is known about the prevalence among children and youth with Down syndrome (CYDS). The purpose of this study was to determine the prevalence of overweight/obesity among CYDS in South West Ireland. Methods: This cross-sectional study measured height and weight of 61 CYDS aged 4-16 years. Body mass index (BMI) was calculated and percentage body fat (PBF) was measured using bio-electrical impendence analysis (BIA). Results: Using the International Obesity Task Force BMI cut-offs, 51.6% of males and 40% of females were overweight/obese compared to 32% and 14.8%, respectively, using PBF. The mean PBF for males was 18.76 versus females 22.38 (p < 0.05). There is a higher prevalence of overweight/obesity, particularly in males, compared to the general population of children. Conclusions: The difference in estimation of overweight/obesity between BMI and BIA has implications for research and clinical practice.