BACKGROUND: During the COVID-19 pandemic, youth in Ontario, Canada experienced a steep rise in mental health concerns. Preventative intervention programs can address the psychological impact of the pandemic on youth and build resiliency. Co-design approaches to developing such programs actively involve young people, resulting in solutions tailored to their unique needs. The current paper details the co-design approach to creating a Preventative Online Mental Health Program for Youth (POMHPY)-a virtually delivered program designed for Ontario youth ages 12 to 25 that promotes mental, physical, and social wellbeing. METHODS: The Participatory Action Research (PAR) framework guided the development of the initiative. Literature reviews were conducted to identify existing evidence-based programs targeting youth. Youth perspectives were primarily gathered via the Youth Advisory Group, comprising a Youth Resilience Coordinator and a Youth Engagement Lead, who contributed to a literature review, surveys, focus groups, and program assets. Community insights were gathered through Community Reference Group (CRG) meetings, which engaged participants from local and provincial organizations, as well as individuals either directly representing or affiliated at arm's length with youth. RESULTS: A review of the current literature highlighted the importance of regular physical activity, social connectedness, good sleep hygiene, and healthy family relationships to emotional wellbeing. Survey findings informed program session length, duration, delivery, and activities. Focus groups expanded on the survey findings and provided an in-depth understanding of youth preferences for program delivery. CRG meetings captured community insights on program refinements to better meet the needs of youth. As such, the development of POMHPY was a collaborative effort among researchers, youth, and community partners. CONCLUSIONS: The findings highlight the value of co-design and PAR-informed approaches in developing youth-targeted online wellbeing programs, providing actionable insights for iterative improvements and future pilot testing. The resulting 6-week program, led by youth facilitators, will focus on teaching mental, social, and physical wellness strategies and skills through various evidence-based, interactive activities.