In student-athletes, the struggle to maintain a balance between athletic and academic performance results in poor mental health. Positive mental health prevents mental health problems. There is very little research on this topic, due to the focus on mental health problems, and the lack of measurement tools. The purpose of this study is to examine the potential of psychological capital (PsyCap) as a psychological resource for the positive mental health of student-athletes. Also, we aim to validate the Psychological Capital Questionnaire (PCQ-24) adapted to the sports context. This research is grounded in the conservation of resources theory, and the broaden-and-build theory. Data were collected using purposive sampling from student-athletes (N = 301). Self-report measures were used and their reliability, validity, and model fitness were evaluated using confirmatory factor analysis. Structural equation modeling was applied for model testing. The competing models for first-order and second-order PsyCap were tested. PsyCap as a synergistic resource has a stronger relationship with positive mental health than each of its dimensions. Furthermore, as an independent psychological resource, only self-efficacy and optimism positively correlated with positive mental health. Additionally, this study shows that the adaptation of PCQ-24 in the sports context was not successful, PCQ-21 is a reliable and valid measure of sport PsyCap. The findings of this study have implications for student-athletes, coaches, sports psychologists, and mental health professionals.