Background: #chatsafe is a youth suicide prevention initiative that aims to support young people to communicate safely online about self-harm and suicide. The #chatsafe guidelines were created in 2018; they are disseminated via the world’s first co-designed youth suicide prevention social media campaign. To date, more than 300 young people have participated in a #chatsafe co-design workshop and the campaign has reached more than 4 million young people globally. Objective: The current study aimed to rigorously evaluate the acceptability, feasibility, and impact on perceived self-efficacy of participating in a #chatsafe co-design workshop. Method: Young people aged 15 to 25 were recruited through youth mental health organizations and grassroots youth programs. To enhance diversity, those typically excluded from suicide prevention activities (e.g., LGBTQIA+, culturally diverse, with self-harm or suicide experience, high school attendees) were specifically invited. Participants completed a Time 1 (T1) survey measuring online communication confidence about self-harm and suicide, and workshop expectations. After a 6-8-hour workshop, a Time 2 (T2) survey assessed workshop acceptability across six domains: appropriateness, attitudes, burden, perceived effectiveness, meeting expectations, and safety. T2 also assessed perceived self-efficacy through increased knowledge, confidence, and skill in online communication about self-harm and suicide. Workshop feasibility was assessed at T1 for reach and engagement and at T2 for perceived support. Findings: Out of 89 interested youth, 45 attended a workshop from November 2022 to February 2023. Participants reported the workshops to be acceptable, with over 90% responding that the workshop was useful, satisfying, and enjoyable, and over 80% reporting emotional and psychological safety. Participants demonstrated enhanced proficiency in using safe language regarding self-harm and suicide (34% confident at Time 1 to 80% at Time 2) and in recognising harmful content online (57% confident at Time 1 to 80% at Time 2). Conclusion: This presentation will share the #chatsafe co-design model evaluated in this study which has been shown to be safe, feasible and acceptable by young people. This model, which includes a step-by-step guide for safe and active involvement, may be implemented at an organisational or community level to support young people’s involvement in self-harm and suicide prevention research and activities.