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Introduction Youth vaping remains a global public health challenge. We will evaluate a pragmatic, theory-driven digital game-based learning intervention (VAPGAMO), delivered in public secondary schools to reduce adolescents’ intention to vape at 3-month follow-up.Methods and analysis Parallel cluster-randomised controlled trial with schools as clusters. Eight public secondary schools in an urban Southeast Asian district (Klang, Malaysia) will be randomised 1:1 to intervention or attention-matched online flash game. The intervention is a single 90 min, facilitator-led module grounded in the Theory of Planned Behaviour. Surveys at baseline, immediately postintervention and 3 months. The primary outcome is intention to vape at 3 months. Secondary outcomes are knowledge, attitudes, injunctive norms and refusal self-efficacy (perceived behavioural control). Prespecified implementation outcomes include acceptability, fidelity, reach, time-on-task and cost per student. Primary analysis will use generalised estimating equations with cluster-robust SEs, adjusted for baseline covariates; the intra-cluster correlation coefficient and design effect will be reported; intention-to-treat will be applied.Ethics and dissemination Universiti Putra Malaysia JKEUPM-2024-887; approvals from the Ministry of Education, Malaysia. Findings will be disseminated in peer-reviewed outlets and shared with education and health authorities to inform school-health programming. De-identified data and code will be made available on publication, subject to approvals.Trial registration number Thai Clinical Trial Registry (TCTR20241222001).