Objective: This study investigated whether and how perceived school climate was associated with adolescent problematic internet use (PIU). Based on two functions of perceived school climate, psychological insecurity and negative peer affiliation were investigated as mediators. Method: A 3-wave longitudinal design (Wave 1 N = 1,365 Chinese adolescents, 53.2% male, M-age = 14.68 years), with 1-year intervals between waves and using adolescent self-report assessment was conducted. Results: Path analysis indicated that (a) Perceived school climate was prospectively associated with PIU 2 years later; (b) When the 2 mediators were analyzed in separate models, both psychological insecurity and negative peer affiliation could play a mediating role in such a prospective relationship; and (c) When the 2 mediators were tested in the same model, two indirect pathways emerged, 1 through psychological insecurity only, and the other involving both psychological insecurity and negative peer affiliation serially. All these effects held even when controlling for initial levels of outcomes and demographic covariates. Conclusions: These findings suggest that positive perceptions of school climate may equip adolescents with psychological security that is instrumental to their peer affiliation, which ultimately decreases their risk of PIU. These findings point to malleable targets for prevention and intervention of adolescent PIU.