Introduction: Adolescents heart transplant recipients experience difficulty with adherence to immunosuppressive medication leading to increased risk of organ rejection, hospitalization, and mortality. Few interventions have been successful to promote medication adherence in adolescent heart transplant patients as most fail to engage the patient in the behavioral change process and support patient–provider communication. The purpose of this study was to explore the nature and degree of in-app communication between adolescent heart transplant recipients and nursing staff during an asynchronous mobile video directly observed therapy intervention. Methods: A content analysis of 894 in-app messages exchanged between 10 adolescent patients and nurses during a 12-week mobile-based intervention was conducted. Two authors used an inductive, iterative process to guide a thematic analysis of the asynchronous in-app messages with high interrater reliability ranging from 81.5% to 100%. Findings: In-app messages fell under 3 broad content categories: (1) interpersonal support and rapport-building, (2) medically related questions and information, or (3) functional information about the intervention and the mHealth app. Results demonstrated the degree to which interpersonal engagement occurred during the intervention, the nature of these exchanges, and their relationship to medication adherence. Conclusions: The study provided insights into the feasibility and benefits of 2-way communication features of the directly observed therapy intervention in promoting engagement and in improving medication adherence among adolescent heart transplant patients. Continued research and clinical focus on patient engagement and impactful aspects of interpersonal communication could aid in the translation of this intervention into standard clinical care at pediatric transplant centers.