Smartphone applications ("apps") can be used to promote health behavior change and expand the reach of behavioral interventions. To date, only a few existing apps have been developed for health promotion among adolescent survivors of childhood cancer. To address this gap, we developed an app-based intervention, using game design characteristics, theory-based behavioral strategies, and assistance from a health coach to motivate health behavior change for adolescent survivors of childhood cancer. This article describes the development and initial feasibility evaluation of the intervention. Using a theoretical framework and an extensive formative process, we developed an app-based game ("Mila Blooms") that promotes healthy eating and physical activity among adolescent survivors of childhood cancer. A single-arm 8-week intervention, using this app-based game, with assistance from a health coach, was conducted among a sample of pediatric cancer survivors ( = 15) to evaluate its initial feasibility for promoting health behavior change. Results from the feasibility evaluation were encouraging. The majority of enrolled participants were retained throughout the 8-week intervention (93.8%). Participant satisfaction feedback indicated positive experiences, related to ease of use and enjoyment of the app. Although there was little evidence for behavior change attributable to the app in this first stage of development, there was a solid demonstration of the viability and appeal of the game features, and there were no adverse side effects. Results provide insights into how gamification can be used to promote health behaviors through an app-based intervention. Mila Blooms holds promise for promoting health behavior change. Lessons learned from our experiences could be useful for the future development and implementation of app-based adolescent health interventions.