The study purpose was to examine whether adolescents who participated in organized physical activity (PA) programs differed from nonparticipants in motivation, social support, and self-efficacy related to PA; PA (min/hr); and sedentary screen time behavior. Thirty-nine 5th-7th grade adolescents participated in organized PA programs; 41 did not. Approximately 56.3% were Black, and 52.5% had annual family incomes <$20,000. Compared to nonparticipants, those who participated reported significantly higher social support (M = 2.32 vs. 3.13, p < 0.001) and fewer hours watching television or movies on a usual weekend day (M = 2.49 vs. 1.59, p = 0.016); and had higher accelerometer-measured vigorous PA (M = 0.58 vs. 1.04, p = 0.009) and moderate-to-vigorous PA (M = 2.48 vs. 3.45, p = 0.035). Involving adolescents in organized PA programs may be important for improving their moderate-to-vigorous PA, vigorous PA, and related psychosocial factors, as well as reducing sedentary screen time behavior.