Adolescent stress-related symptoms (anxiety, depression) have dramatically increased in the past decade. How can adolescents be better helped to adequately cope with stress? Novel easy-to-implement, and low-cost interventions are needed to complement existing programs, whose effectiveness is often limited. Drawing on prior work on adolescent physiological plasticity, we propose that enhancing parasympathetic nervous system activity via resonant breathing (RB) – a slow-paced breathing technique – may be highly beneficial to increase stress resilience. To investigate both the immediate and longer-term benefits of RB on momentary affect and stress-related symptoms, we propose to deliver RB in adolescents’ daily life via a mental Health app, using an ecological momentary intervention (EMI). Specifically, in a randomized controlled trial 250 stress-vulnerable adolescents will be assigned to 2-week RB EMI or an active control condition with pre- and post-measurements of mental health, psychophysiology, daily life emotion dynamic (via ecological momentary assessment) and objective measures of sleep. Throughout the project, stakeholders (adolescents, teachers, parents, organisations), will be involved in focus groups to identify the best conditions for a feasible intervention, to evaluate the process and to explore future implementations. Together, this research will offer unique knowledge for tailoring intervention and prevention programs in several youth contexts.