Background: Rates of adolescent HIV and unintended pregnancy in southern Africa are amongst the highest in the world. Gender-transformative interventions that address underlying gender inequalities and engage both males and females have been emphasised by the World Health Organisation, amongst others, to target prevention. However, few such gendertransformative interventions have been rigorously developed or evaluated. Objective: To expedite potential impact and reduce development costs, we conducted a needs assessment to inform the co-design, in consultation with local stakeholders, of adapted versions of an existing gender-transformative Relationships and Sexuality Education intervention for use in South Africa and Lesotho. Methods: Adaptation of the intervention was guided by a modified version of Intervention Mapping (IM). This process involved consultation with separate adolescent, community and expert advisory groups and a collaboratively conducted needs assessment, which drew on focus groups with adolescents (8 groups, n = 55) and adults (4 groups, n = 22) in South Africa and Lesotho, and was informed by our systematic review of the literature on the determinants of condom use among adolescents in the region. Results: The findings clarified how the intervention should be adapted, which individual-and environmental-level determinants of condom use to target, and actions for facilitating successful adoption, evaluation and implementation in the new settings. Conclusions: The IM approach allows for a systematic appraisal of whether components and processes of an existing intervention are appropriate for a new target population before costly evaluation studies are conducted. The findings will be of interest to those wishing to rigourously develop and evaluate gender-transformative interventions engaging men to improve health for all.