BACKGROUND: In the global debate around transactional sex little attention has concentrated on Brazil, despite ranking fourth globally in absolute number of girls married or co-habiting by the age of 15 years, and evidence showing that these unions often begin as age-disparate transactional sex (ADTS). This article contributes to filling this gap by exploring the personal beliefs and social norms related to ADTS in urban (favela) communities of Rio de Janeiro, Brazil between adult men (> 18 years) and girls and adolescents (G/A) (< 18 years) with a minimum 5-year age disparity. The primary objective of this study was to identify the social norms that promote and prevent ADTS, and the dynamics between individual beliefs and social norms, to provide contextualized recommendations to prevent ADTS in this setting. METHODS: An exploratory, sequential, mixed-methods design was used, starting with a qualitative phase that included semi-structured, in-depth interviews and focus groups, and a subsequent quantitative phase comprising of a community survey. The items for the quantitative questionnaires were developed based on the qualitative results. RESULTS: Mixed methods results indicate that in these communities ADTS is normalised and not considered exploitative. We identified three themes related to the reasons ADTS occurs: girls' responsibility, male desires and benefits of ADTS. Men's role in ADTS was largely minimised because of a general acceptance of a notion of masculinity characterised by hypersexuality and lack of impulse control. Individual beliefs, however, did not tend to align with these social norms. CONCLUSIONS: In this study, personal beliefs and social norms often did not align, suggesting that initiatives working to change personal or attitudes regarding ADTS may not lead to meaningful change in ADTS behaviours, and social norms interventions may be more effective. Our findings reinforce the need to develop programs tailored to local understandings of ADTS, targeting not only girls but also a wide range of actors. Interventions could also consider the structural factors acting in local and global contexts that promote or prevent ADTS.