BACKGROUND: Type 1 diabetes mellitus (T1DM) afflicts approximately 154,000 people under the age of 20 in the United States. Most people with T1DM are diagnosed at a young age, and parents have to take on the responsibility of T1DM management. Eventually, the child must begin to transition to self-management. Adolescents often struggle to take on responsibility for all the necessary tasks to successfully self-manage their T1DM. In fact, approximately three-quarters of adolescents are not achieving American Diabetes Association-recommended glycated hemoglobin (HbA1c) targets. This lack of adherence can lead to negative health outcomes. OBJECTIVE: The goals of this interdisciplinary proposal are as follows: (1) to develop a unique and theory-driven technology using a mobile phone app to promote self-management behaviors for adolescents aged 10-15 years with T1DM and their parents and (2) to explore the feasibility and impact of the self-management mobile app. METHODS: This study has two phases: app development and pilot testing. In the app development phase, the app will be conceptualized and a prototype will be tested. In Phase 2, the mobile app will undergo pilot testing to determine its feasibility and impact on diabetes self-management. RESULTS: The pilot test was launched in September 2017. Data collection for the final pilot test is underway, and results are forthcoming. CONCLUSIONS: Adolescents with T1DM and their parents can have a difficult time managing the transition of diabetes care. It is hoped that this app can help. The focus groups and prototype testing have indicated promising outcomes of app use. TRIAL REGISTRATION: ClinicalTrials.gov NCT03436628; https://clinicaltrials.gov/ct2/show/NCT03436628 (Archived by WebCite at http://www.webcitation.org/72tHXTE2Z). INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR1-10.2196/10803.