This study examined the effect of cognitive demand on situational interest and performance using running tasks in physical education. Adolescents (N = 179) participated in a field study involving three different levels of cognitive demand. Running performances and situational interest were measured four times with a testing interval of seven days. Results from multivariate analyses of covariance suggested that cognitive demand had a significant effect on situational interest and running performances. Higher cognitive demand led to higher levels of situational interest. Situational interest was weakly or moderately correlated with running performances (|r| = 0.17-0.40), yet students reported significantly lower performance when the perceived interest was the highest. These findings suggest the importance of designing instructional tasks with moderate cognitive demand to optimally preserve situational interest and performance in physical education. The findings also help further conceptualise the relations among cognitive demand, situational interest and performance.