The widespread adoption of smartphones among adolescents has led to a growing concern regarding its potential impact on their mental health. This longitudinal study aims to examine the relationship between long-term smartphone use and adolescent mental health outcomes over a significant period. A cohort of adolescents aged 13 to 17 years was recruited from schools across diverse socio-economic backgrounds. Baseline data on smartphone usage patterns and mental health indicators, including anxiety, depression, stress, and overall well-being, were collected using standardized self-report questionnaires. Over the course of three years, participants were regularly assessed at six-month intervals to capture changes in smartphone use and mental health outcomes. Additionally, various confounding factors such as age, gender, socioeconomic status, and pre-existing mental health conditions were considered during data analysis. Preliminary results from the first year suggest a positive correlation between the duration of daily smartphone use and higher levels of anxiety and stress. Moreover, a negative association was observed between smartphone use and overall well-being and life satisfaction. Through a comprehensive analysis of the longitudinal data, this study aims to provide deeper insights into the potential causality of these associations. It will explore whether increased smartphone use leads to deteriorating mental health or whether adolescents with pre-existing mental health issues are more likely to engage in excessive smartphone use as a coping mechanism. The findings of this study will have significant implications for policymakers, educators, parents, and mental health professionals, as they seek to promote responsible smartphone usage and address potential mental health challenges among adolescents. By identifying the key factors contributing to the link between smartphone use and mental health, this research aims to provide evidence-based recommendations for developing effective intervention strategies and support systems to mitigate adverse effects and enhance adolescent well-being in the digital age.