Food Game is a gamified school-based intervention that aims to promote healthier dietary choices (i.e., adherence to the Mediterranean diet) and more sustainable behaviors among high school students in an urban area in Northern Italy. The program consists of an offline and online competition in which groups of students participate in peer-led activities to design and communicate products promoting health and sustainability. This study aimed to examine how the program works in practice, understand its mechanisms of change and assess any variation in student outcomes. A mixed methods process evaluation was conducted. Students completed a three-wave longitudinal survey and participated in focus groups. Program staff and teachers were also interviewed. Qualitative and quantitative data analyses indicate that Food Game?s gamification strategy was successful in engaging students, who felt stimulated by the game, its embedded competition and the self-organized group work. Although no significant change in adherence to the Mediterranean diet was found, pro-environmental behaviors, attitudes and perceived peer approval on healthy eating increased over time. Findings provide preliminary support for Food Game as an acceptable and engaging intervention though there is yet not sufficient evidence that it is also promotes healthier and more sustainable behaviors. The results contribute to the limited evidence base for offline gamified school-based interventions and offer insight into the applicability of the gamification approach as a key motivational strategy in other prevention settings and contexts. Suggestions on how to strengthen the program include dosage increase and addition of activities to promote change on relevant school and community factors.