The purpose of this study was to determine the effect of adding game design elements (goal setting, feedback, and external rewards) to an evidence-based physical activity intervention to increase in-class physical activity participation (intensity of activity breaks performed). Nine third- through sixth-grade classrooms ( = 292 students) in one elementary-middle school in Detroit, Michigan (49% female, 95% nonwhite; 80% qualified for free/reduced lunch) participated in this 20-week intervention where teachers implemented 5 × 4 min moderate-to-vigorous activity breaks per day. Gamification of activity breaks occurred during weeks 13-20 of the intervention and included the use of game design elements and classroom goals for activity break intensity. Moderate-to-vigorous physical activity (MVPA) during activity breaks was measured via direct observation. There was a significant effect of intervention with a 27% increase in student MVPA participation during the gamified intervention weeks compared with the standard intervention weeks ( = 0.03). Gamification of activity breaks resulted in 55% (compared with 25% during the standard intervention) of students accumulating approximately 20 min of health-enhancing physical activity per day in their classroom. These findings provide preliminary evidence that gamifying activity breaks may be an important strategy for increasing student participation in classroom activity breaks.