The rapid proliferation of digital technologies has raised increasing concerns about Internet addiction (IA) and its adverse effects on adolescents' mental health, particularly the onset of depressive symptoms. While prior research has established a robust association between IA and depression, limited attention has been given to potential protective factors that may buffer this relationship. Grounded in educational psychology, the present study examined the moderating role of Social and Emotional Learning (SEL), a framework that emphasizes emotional regulation, interpersonal competence, and responsible decision-making. Data were drawn from the Taiwan Assessment of Student Achievement Longitudinal Database (TASAL), with a nationally representative sample of secondary school students (N = 8,739). IA, SEL, and depression were assessed with validated instruments including the Chinese Internet Addiction Scale, the WCSD Social and Emotional Competency Assessment, and the CES-D. Descriptive analyses, Pearson correlations, and moderation tests using PROCESS macro Model 1 were conducted. Results indicated that IA was positively associated with depressive symptoms, yet this effect was significantly attenuated among students with higher SEL competencies. Specifically, adolescents with low SEL exhibited a steep increase in depressive symptoms as IA increased, whereas those with high SEL showed a buffered effect with substantially weaker symptom elevation. These findings provide large-scale empirical support for SEL as a protective factor in adolescent development, highlighting its potential to mitigate the psychological risks of excessive Internet use. The study underscores the value of embedding SEL programs in school curricula to enhance resilience, reduce vulnerability to digital-related mental health challenges, and foster sustainable well-being among youth.