BACKGROUND:teen Mental Health First Aid (tMHFA) is a classroom-based training programme for students aged 15-18 years to improve supportive behaviours towards peers, increase mental health literacy and reduce stigma. This research evaluated tMHFA by comparing it to a matched emergency Physical First Aid (PFA) training programme. METHODS:A cluster-randomised crossover trial matched four public schools in two pairs and then randomised each to first receive tMHFA or PFA for all Year 10 students. In the subsequent calendar year, the new Year 10 cohort received the opposite intervention, giving eight cohorts. Online surveys were administered at baseline and 1 week post-training, measuring quality of first aid intentions, mental health literacy, problem recognition and stigmatising beliefs, towards fictional adolescents with depression and suicidality (John) and social anxiety (Jeanie). RESULTS:A total of 1942 students were randomised (979 received tMHFA, 948 received PFA), 1605 (84%) analysed for the John vignette at baseline and 1116 (69% of baseline) provided post-training data. The primary outcomes, 'helpful first aid intentions' towards John/Jeanie, showed significant group-by-time interactions with medium effect sizes favouring tMHFA ( ds = 0.50-0.58). Compared to PFA, tMHFA students also reported significantly greater improvements in confidence supporting a peer ( ds = 0.22-0.37) and number of adults rated as helpful ( ds = 0.45-0.46) and greater reductions in stigmatising beliefs ( ds = 0.12-0.40) and 'harmful first aid intentions' towards John/Jeanie ( ds = 0.15-0.41). CONCLUSIONS:tMHFA is an effective and feasible programme for increasing supportive first aid intentions and mental health literacy in adolescents in the short term. tMHFA could be widely disseminated to positively impact on help seeking for adolescent mental illness.