Background: Adequate skill levels of gross motor capacity affect activities of daily living, participation in recreational activities and general physical activity levels of youths (7–21 years). Most studies of typically developing youths have reported significant negative relationships between gross motor capacity and body mass index. The latter findings are especially of concern for youths with intellectual disabilities in that it has been estimated that 61% of children and 66% of adolescents were classified as overweight/obese. Therefore, the purpose of this study was to determine the strength of the relationship between body mass index and gross motor capacity among youths with mild to moderate intellectual disability (ID). Methods: Components of the Bruininks–Oseretsky Test of Motor Proficiency (BOT-2) were used for designated aspects of gross motor capacity: six items for upper limb coordination (ULC); seven items for balance (BAL); six items for bilateral coordination (BLC); and one item for agility (A-2). Participants consisted of 654 youths (438 men), ages 8–21 years with ID. Participants were divided into pre-puberty and post-puberty men (post ≥12 years) and women (post ≥10 years of age). Body mass index (BMI, kg/m2) was determined by height and weight measurements on the day of testing. A Kendall's tau correlation coefficient (τ) was used to determine the strength of the relationship between body mass index and gross motor capacity (BOT-2 test scores). Results: The τ values for both pre-puberty and post-puberty for all BAL, BLC, A-2 tests and for three of the six ULC tests were negligible to very weak (τ = 0 to ±0.19). Higher τ values were seen for pre-puberty youths in three of the ULC tests, but they fell within the weak range (τ < 0.24). When combining all pre-puberty and post-puberty participants, τ values were in the negligible to very weak range for all tests. Conclusion: The strength of relationship between body mass index and gross motor capacity as measured by the BOT-2 subtest item scores used in this study is very weak and suggests that they are not clinically relevant. © 2024 John Wiley & Sons and MENCAP.