In Tanzania, adolescents face both undernutrition and a rising prevalence of obesity. This study details the co-design and prototyping of a school-based nutrition intervention among adolescents in Dar es Salaam, Tanzania. The study followed human-centered design principles. Four co-design workshops were conducted with stakeholders to develop the intervention, which was prototyped in two schools with students aged 13–15 years and school staff. The intervention’s acceptability, appropriateness, and feasibility were assessed using mixed-methods. Workshops identified four key challenges for action: poor nutritional status, limited nutrition awareness, low physical activity, and poor sanitation and hygiene practices. The resulting intervention—COMPASS, a COMprehensive health and nutrition Program for AdolescentS in Schools—integrates twelve nutrition activities designed for delivery at whole-school, group, and individual levels by trained teachers. Qualitative feedback was positive, with students appreciating enhanced nutrition knowledge and health practices and feeling empowered as change agents. A post-intervention quantitative survey of 220 students showed high levels of enjoyment (93.6