BackgroundAdolescent mental health has become a public health concern as 10–20% of adolescents have experiences with mental health problems. Improving mental health education is critical to reducing stigma and improving access to appropriate care when needed. Here we examine the impact of a mental health literacy programme (Guide Cymru) in young adolescents in the UK. A randomised controlled trial assessed the effectiveness of the Guide Cymru intervention.MethodA total of 1,926 pupils (860 males and 1066 females) aged 13–14 (year 9) took part in the study. The secondary schools were randomised into the active and control arms of the study. Teachers in the active arm of the study were trained on the Guide Cymru and then delivered the intervention to their pupils. Pupils in the active groups received six modules of mental health literacy (the Guide Cymru), and control schools received teaching as usual. Mental health literacy across several domains (e.g., knowledge, stigma, help-seeking intentions) were assessed both before and after the intervention. Data collection for the randomised controlled trial ran from September 2019 to March 2020. Multi-level modelling analysis was conducted to account for the clustered nature of the design.ResultsAll aspects of mental health literacy, including mental health knowledge (g= 0.32), good mental health behaviours (g= 0.22), mental health stigmas (g= 0.16), intentions to seek help (g= 0.15), and avoidant coping (g= 0.14) improved after completing the Guide Cymru programme (ps < .001).DiscussionThe current study presents evidence for the Guide Cymru’s effectiveness in improving secondary school pupils' mental health literacy. We demonstrate that providing teachers with appropriate resources and training to deliver the Guide Cymru programme within their classrooms can improve the mental health literacy of pupils. These findings have important implications for the beneficial impacts the secondary school system can have on reducing the burden of mental health problems at a critical point in a young person’s life.Trial registrationISRCTN15462041. Registered 03/10/2019.