ABSTRACT Adolescents with pulmonary arterial hypertension (PAH) have impaired exercise capacity and reduced quality of life. Current therapies only modestly improve exercise capacity and are often prohibitively expensive, highlighting the need for additional, cost-effective interventions. Multiple studies have demonstrated that increasing physical activity is highly efficacious in adult PAH, resulting in six-minute walk distance (6MWD) improvement that exceeds the effect of medications. However, protocols require inpatient rehabilitation, making them impractical and poorly scalable to the adolescent PAH population. Moreover, cardiopulmonary rehabilitation is poorly reimbursed, and facilities are not easily available or accommodating of adolescents. Recently, in adults, we completed the first mobile health (mHealth) intervention in the PAH population, demonstrating the effectiveness of linking real time activity tracking with a “smart” behavioral coaching intervention, using personalized, encouraging content based in behavioral change theory (e.g. feedback loops and habit formation). After 12 weeks, the intervention arm took 1019 more steps per day than the control arm (adjusted for age, sex, functional class, and baseline step counts), with <5% dropout. In secondary analyses, we observed significant improvements in quality of life (QOL) and right ventricular function. But, adolescents may respond differently to remote real time activity monitoring and behavioral coaching, demanding optimization of our approaches. We hypothesize that a mHealth intervention specifically designed for the needs of an adolescent population with PAH is feasible and will increase daily step counts. We propose a randomized pilot trial of smart texts versus usual care for 12 weeks using personalized, adaptive step count targets and text messages that leverage age-appropriate behavioral psychology. Using a fully remote study without in-person visits, 50 participants will wear a Fitbit device