This study aimed to investigate the influence of a team leader's personality on students' academic performance and explore the underlying mechanism. Three hundred thirty-eight university students who were enrolled in a Happy Psychology curriculum participated in the survey. Hierarchical linear regression was used to test the hypotheses, with academic performance as the outcome, the team leader's personality as the independent variable, group participation as the mediator, and gender diversity as the moderator. The results showed that the neuroticism and conscientiousness of the team leader had a direct or indirect effect (through group participation) on university students' academic performance and that gender diversity moderated the direct or indirect effect of group participation on students' academic performance. These findings could broaden and deepen the understanding of the influence of team leaders' personality on students' academic performance in problem-based classes. A team leader's personality and gender diversity should be considered in team construction.