BACKGROUND: Young adult offenders represent a third of the UK prison population and are at risk of poor health outcomes including drug and alcohol misuse, self-harm and suicide. Court diversion interventions aim to reduce the negative consequences of formal criminal justice sanctions and focus resources on addressing the root causes of offending. Although diversions are widely used, evidence of their effectiveness has not yet been established. Hampshire Constabulary, working together with local charities, have developed the Gateway programme, an out-of-court intervention aimed at improving the life chances of young adults. Issued as a conditional caution, participants undertake a health and social care needs assessment, attend workshops encouraging analysis of own behaviour and its consequences and agree not to re-offend during the 16-week caution. METHODS: This is a pragmatic, multi-site, parallel-group, superiority randomised controlled trial with a target sample size of 334. Participants are aged 18-24, reside in Hampshire and Isle of Wight and are being questioned for an eligible low-level offence. Police investigators offer potential participants a chance to receive the Gateway caution, and those interested are also invited to take part in the study. Police officers obtain Stage 1 consent and carry out an eligibility check, after which participants are randomised on a 1:1 basis either to receive Gateway or follow the usual process, such as court appearance or a different conditional caution. Researchers subsequently obtain Stage 2 consent and collect data at weeks 4 and 16, and 1 year post-randomisation. The primary outcome is the Warwick-Edinburgh Mental Well-being Scale (WEMWBS). Secondary outcomes include health status, alcohol and drug use, recidivism and resource use. The primary analysis will compare the WEMWBS score between the two groups at 12 months. DISCUSSION: This pioneering trial aims to address the evidence gap surrounding diversion in 18-24-year-olds. The findings will inform law enforcement agencies, third sector organisations, policymakers and commissioners, as well as researchers working in related fields and with vulnerable target populations. TRIAL REGISTRATION: International Standard Randomised Controlled Trial Register ( ISRCTN 11888938 ).