Recent statistics reveal a concerning rise in internalizing disorders such as anxiety and depression among adolescents, highlighting the need for innovative, cost-effective interventions to complement existing treatments. Adolescents’ heightened neural and physiological plasticity presents a unique opportunity to leverage bottom-up approaches that directly target autonomic regulation. This project proposes slow-paced breathing (SPB)—a technique to enhance heart rate variability (HRV)—as a promising intervention to improve mental well-being in clinical adolescents. To investigate the immediate and long-term benefits of SPB, I will conduct a randomized controlled trial with 100 adolescents experiencing internalizing symptoms. Participants will engage in a four-week SPB intervention delivered via a mobile app, with ecological momentary interventions used to capture real-time emotional and behavioral changes. This study will extend prior research by examining the underlying mechanisms of SPB, including its effects on HRV, Default Mode Network activity (via EEG), and emotion dynamics in daily life. The project also incorporates stakeholder involvement, including adolescents, parents, and clinicians, to evaluate the feasibility of integrating SPB into clinical care. Together, this research aims to establish SPB as a scalable, non-invasive tool, advancing intervention strategies for adolescent mental health and contributing to prevention and treatment programs.