Adolescents with type 1 diabetes (T1D) experience high rates of diabetes distress, which negatively influence self-management and glycemic control. Building on effective positive psychology interventions to improve adherence in adults, as well as our pilot work to adapt these interventions for adolescents, we developed a positive psychology intervention for adolescents with T1D. The goal of THR1VE! is to reduce diabetes distress in adolescents with T1D and improve their diabetes outcomes. This multi-site randomized controlled trial compares a Diabetes Education + text-message-based Positive Affect intervention, to a Diabetes Education control condition. In the ongoing trial, we are evaluating the effects of the intervention on adolescents' diabetes distress, self-management, and glycemic control. This paper describes the rationale, trial design, and methodology of the THR1VE! Study.