Gamification is an increasingly popular form of health intervention but its efficacy remains elusive due to a lack of clarity in its conceptualization and operationalization. This study aimed to isolate and determine the direct causal effect of one of the most popular game elements used in gamified interventions, the points-based reward system, on physical activity (PA) in children. A 72-hour field study with children aged 9 to 13 (N = 67) was conducted using a digital PA intervention featuring a virtual dog, with and without a points-based reward system. PA was assessed with an activity monitor, and overall PA, three levels of PA intensity, and PA strategies during the 3-day intervention were measured. Guided by self-determination theory, the impact of the points-based reward system on children's basic psychological needs was also investigated. Results indicated that the points-based reward system briefly increased PA engagement but did not significantly affect overall PA over time. When given equal number of points regardless of intensity, children approached the PA intervention strategically by engaging in significantly more light intensity and significantly less vigorous intensity PA than children who did not receive points. Results also suggested that the points-based reward system might promote perceptions of relatedness with the virtual agent featured in the gamified intervention.