Background <p>Children and adolescents with intellectual and developmental disabilities (IDD) often experience reduced health-related physical function. However, the correlation between decline in physical function and reduced limb skeletal muscle mass among adolescents with IDD is not yet clear. This study aimed to clarify the relationship between limb skeletal muscle mass and physical function in adolescents with IDD.</p> Methods <p>This short-term longitudinal observational study included 53 adolescents (aged 12–18 years) with IDD from special needs schools who attended community sports classes in Kobe City, Japan. The assessment included body composition analysis (skeletal muscle mass index: SMI) and physical fitness tests (handgrip strength, standing long jump, sit-ups, sit-and-reach test, six-minute walk test, pulmonary function tests), complemented by physical activity level questionnaires. </p> Results <p>Significant positive correlations were observed between SMI and handgrip strength, as well as between SMI and standing long jump performance. The physical fitness assessment showed high reliability upon re-measurement after 4 weeks. Additionally, multiple linear regression analyses considering SMI as the dependent variable and adjusting for age, sex, body mass index, and physical activity level showed significant positive associations for handgrip strength and standing long jump.</p> Conclusions <p>This study revealed significant associations between skeletal muscle mass index and physical fitness indicators, such as handgrip strength and standing long jump, in adolescents with intellectual and developmental disabilities. These measures may be effective indicators for assessing limb skeletal muscle development in this population.</p>