BACKGROUND: Despite effective biomedical tools, HIV remains the largest cause of morbidity/mortality in South Africa - especially among adolescents and young people. We used community-based participatory research (CBPR), informed by principles of social justice, to develop a peer-led biosocial intervention for HIV prevention in KwaZulu-Natal (KZN). METHODS: Between March 2018 and September 2019 we used CBPR to iteratively co-create and contextually adapt a biosocial peer-led intervention to support HIV prevention. Men and women aged 18-30 years were selected by community leaders of 21 intervention implementation areas (izigodi) and underwent 20 weeks of training as peer-navigators. We synthesised quantitative and qualitative data collected during a 2016-2018 study into 17 vignettes illustrating the local drivers of HIV. During three participatory intervention development workshops and community mapping sessions, the peer-navigators critically engaged with vignettes, brainstormed solutions and mapped the components to their own izigodi. The intervention components were plotted to a Theory of Change which, following a six-month pilot and process evaluation, the peer-navigators refined. The intervention will be evaluated in a randomised controlled trial ( NCT04532307 ). RESULTS: Following written and oral assessments, 57 of the 108 initially selected participated in two workshops to discuss the vignettes and co-create the Thetha Nami (`talk to me'). The intervention included peer-led health promotion to improve self-efficacy and demand for HIV prevention, referrals to social and educational resources, and aaccessible youth-friendly clinical services to improve uptake of HIV prevention. During the pilot the peer-navigators approached 6871 young people, of whom 6141 (89%) accepted health promotion and 438 were linked to care. During semi-structured interviews peer-navigators described the appeal of providing sexual health information to peers of a similar age and background but wanted to provide more than just "onward referral". In the third participatory workshop 54 peer-navigators refined the Thetha Nami intervention to add three components: structured assessment tool to tailor health promotion and referrals, safe spaces and community advocacy to create an enabling environment, and peer-mentorship and navigation of resources to improve retention in HIV prevention. CONCLUSION: Local youth were able to use evidence to develop a contextually adapted peer-led intervention to deliver biosocial HIV prevention.