Although considerable research has examined the antecedents of adolescents’ pornography use (PU), little data exist on the underlying mechanisms of problematic pornography use (PPU) in adolescence. To bridge this gap, the current longitudinal study examined the long-term associations between adolescent boys’ PU frequency, PPU, and theory-based predictors of PPU. Testing three conceptual models (the moral incongruence, out of control, and mood management models), we examined the relative contributions of impulsivity, depression/anxiety symptoms, and religiosity to PPU, controlling for PU over time. The analysis was conducted using a panel sample of 337 Croatian men (M_age = 15.9 years, SD = 0.52). Latent growth curve modeling showed that baseline levels of, and subsequent growth in PU predicted later higher levels of PPU, irrespective of participants’ religiosity, negative emotions, and impulsivity. Baseline higher levels of negative emotions and impulsivity predicted higher levels of PPU three years later, even after controlling for the growth in PU over time. Religiosity moderated the association between changes in PU and PPU. Growth in PU resulted in higher PPU among more religious adolescents, compared to their less religious peers. This study’s findings support all three conceptual models explaining PPU in adults and may guide clinical work with adolescents seeking help for PPU.