ABSTRACT: Many children are not sufficiently physically active. This study uses a quasi-experimental design to evaluate whether participation in a before-school physical activity program called Build Our Kids' Success (BOKS) increases physical activity. Participants (n = 426) were students in Fall, 2016 enrolled in BOKS programming and matched non-BOKS control students from the same grades (Kindergarten-6) and schools in Massachusetts and Rhode Island. Analyses conducted in 2017 examined differences between children in BOKS versus controls in total daily steps, minutes of moderate-to-vigorous (MVPA), vigorous (VPA), and total physical activity (TPA) assessed via Fitbit Charge HR™ monitors. Additional analyses compared physical activity on program days and non-program days. Students (mean age = 8.6 y; 47% female, 58% White, Non-Hispanic) wore monitors an average of 21.7 h/day on 3.2 days during the school week. Compared with controls, on BOKS days, BOKS participants accumulated more steps (1147, 95% confidence interval (CI): 583-1712, P < 0.001), MVPA minutes (13.4, 95% CI: 6.6-20.3, P < 0.001), and VPA minutes (4.0, 95% CI: 1.2-6.7, P = 0.005). Across all school days, BOKS participants accumulated more total steps than controls (716, 95% CI: 228-1204, P = 0.004). Compared to days without BOKS programming, on BOKS days, BOKS participants accumulated more steps (1153; 95% CI: 841-1464, P < 0.001) and daily minutes of MVPA (8.8, 95% CI: 5.3-12.2, P < 0.001), VPA (3.0, 95% CI: 1.6-4.5, P < 0.001), and TPA (20.8, 95% CI: 13.6-28.1, P < 0.001). BOKS programming promotes engagement in additional accumulated steps during the school week and physical activity on days that students participate. Clinical Trial Registration: www.ClinicalTrials.gov, NCT03403816, available at: https://clinicaltrials.gov/ct2/show/NCT03403816?term=NCT03403816&rank=1.