Background: Youth experience a decrease in physical activity (PA) and an increase in sedentary time during adolescence. Better understanding of factors associated with activity levels during preadolescence may inform interventions to minimize decline. This study compared the association of self-efficacy for PA, parent support for PA, and peer support for PA with moderate-to-vigorous PA (MVPA) and sedentary time among 8- to 12-year-old children with body mass index ≥75th percentile. Methods: This study analyzed baseline data from a school-based healthy weight management intervention trial, conducted in metropolitan Minnesota. Self-efficacy for PA, parent support for PA, and peer support for PA were measured by child survey using reliable tools. MVPA and sedentary time were measured using accelerometer. Results: Participants included 114 children; mean age was 9.4 (0.9) years, 51% were females, 55% received public assistance, and 57% were racial/ethnic minorities. Self-efficacy for PA was positively associated with moderate to vigorous PA for girls (β = 1.83, P < .01) and inversely with sedentary time for the total sample (β = −7.00, P = .03). Parent support for PA was positively associated with sedentary time for girls (β = 9.89, P = .04) and the total sample (β = 7.83, P = .04). Conclusions: Interventions for preadolescents with elevated body mass index may improve activity levels by increasing self-efficacy for PA.