Objective: This study examines the efficacy of incorporating exergaming into physical education (PE) lessons among children and pre-adolescents in influencing the social cognitive factors and behaviors of physical activity (PA), based on the theory of planned behavior (TPB). Design: We conducted a six-week study centered on a 2 (exergaming: PE lesson with Wii vs. PE lesson without Wii) × 2 (age group: 10 years-old children vs. 12 years-old pre-adolescents) between-subject factorial design. Method: 1112 participants were randomly assigned to the study conditions. At the end of the six-week program, participants completed a survey consisting of measures from the TPB variables. Results: Exergaming significantly influenced PA attitude, subjective norm, intention, and strenuous exercise behavior, with participants in the Wii-incorporated PE lesson more likely to emerge with more positive beliefs and behaviors. Age significantly influenced outcome variables, with the effect of exergaming more pronounced among children than pre-adolescents in attitude, moderate and mild exercise behaviors. Conclusions: Incorporating exergaming into PE lessons can be more effective than regular PE in enhancing PA beliefs and behaviors, particularly among younger children.