Abstract: Background: Vocational students have an increased risk to engage in health-risk behaviors compared to same-aged peers. To date, evidence-based digital prevention approaches that address multiple health-risk behaviors are rare. Method: The randomized-controlled trial (RCT) “Prevention of at-risk substance and Internet use disorders among vocational students” (PARI) investigates the efficacy of an app-based prevention approach compared to a waitlist-control condition. The aim is to prevent substance-related and behavioral addictions and improve life skills. An existing app (ready4life) was adapted under consideration of focus groups with teachers, prevention experts, and students. A Delphi expert group rated the quality of the approach. The efficacy of the modified ready4life app is currently being tested in a RCT. The proactive recruitment takes place in German vocational schools. After participating in an app-based screening (T0), participants get individualized feedback and will be cluster-randomized per class to the intervention group (IG; n=1.250) or control group (CG; n=1.250). The IG chooses two out of six modules: Social competence, stress management, cannabis, tobacco, alcohol, social media/gaming. The CG receives information on how to improve health behaviors. Follow-ups are conducted after 6 months (T1) and 12 months (T2). Conclusion: This RCT provides data on a multibehavioral prevention approach for vocational students. Final results are expected in 2023.