<p>Academic stress is a significant source of psychological strain for adolescents in South Korea and is widely recognized as a risk factor for depression. This study examined the relationship between academic stress and depression in adolescents and explored whether physical activity and smartphone usage serve as moderating factors. Data were collected from 920 students across eight middle schools and seven high schools in South Korea (53.9% female students and 56.5% high school students). A multiple additive moderation model was analyzed using Hayes’s PROCESS macro (Model 2). Results showed that academic stress significantly predicted higher levels of depression. Physical activity was associated with lower levels of depression and significantly moderated the relationship between academic stress and depression, buffering its negative effects. In contrast, smartphone usage was directly associated with higher levels of depression, though it did not yield a statistically significant moderating effect. These findings underscore the critical role of physical activity as a proactive coping resource that cultivates internal psychological resilience, thereby mitigating depression among adolescents facing academic stress.</p>