Background:Previous research has shown a positive relationship between activity-related social support provided by parents and peers and adolescents’ physical activity. However, more information is needed on whether activity-related social support differs by sociodemographic characteristics. The purpose of this study was to investigate differences in activity-related social support in a sample of adolescents, by characteristics such as age, gender, socioeconomic status (SES), ethnicity, and physical activity level and to determine which characteristics are the most important predictors of activity-related social support.Methods:Information was provided by 578 boys and 588 girls (11–14 years) on demographic factors, physical activity, and activity-related support. ANOVA, correlations, and multiple regression were performed to address the purposes of the study.Results:Boys, White British, younger, more physically active, and high-SES adolescents perceived more support for physical activity. Age predicted all types of support excluding peer support; ethnicity predicted mother logistic support and sibling support; gender predicted peer support, father explicit modeling, and father logistic support; and SES predicted mother and father logistic support.Conclusions:Families and peers of adolescents who are female, from Black and minority ethnic groups, older, of low-SES, and less active should be targeted for intervention.