The benefits and challenges of integrating exergames in schools to promote young children‘s physical activity and interest have been evident. Yet, such investigations remain unexplored among older adolescents. Thus, the purpose of this study was to examine the relationships between high school students’ situational interest and physical activity levels during exergame play, as well as investigate the gender and grade differences on the study variables. A total of 60 (32 girls; Mean = 16.17 years, SD = 1.17) 10th through 12th grade students participated in one session of Reflex Ridge, part of the Xbox 360 Kinect Adventures game. Students‘ physical activity levels were measured by Actigraph accelerometers with percentages of time spent in sedentary, light physical activity and moderate-to-vigorous physical activity as the outcome variables. They also responded to the validated Situational Interest Scale (measuring novelty, challenge, attention demand, exploration intention, and instant enjoyment) at the end of the session. Regression and path analysis indicated that attention demand was a significant positive predictor for moderate-to-vigorous physical activity (ß = 0.41, p < 0.05 accounting for 16.7% of the variance) while novelty was a marginally significant positive predictor for light physical activity (ß = 0.25, p < 0.06 accounting for 13.4% of the variance). Multivariate analysis of variance revealed no gender effect on sedentary, light physical activity and moderate-to-vigorous physical activity, and that 11th grade students spent significantly higher percentages of time in moderate-to-vigorous physical activity than 10th and 12th grade students (F (6,104) = 2.51, p < 0.05). Findings suggested high school students displayed higher moderate-to-vigorous physical activity when they felt the activity demanded greater attention but demonstrated higher light physical activity when they deemed the activity only provided novelty. The motivational power of exergaming may be an opportunity to intervene and attenuate the increasingly sedentary daily activities currently engaged in by adolescents. However, physical educators should consider the motivational power of attention (i.e., cognitive) demand to physically engage high school students in exergaming experiences.