Premarital sex (PMS) among adolescents and young people (AY) is reported to be increasing worldwide. Research to date has tended to focus on analysing the prevalence of PMS among AY, and the individual characteristics associated with PMS, rather than pursuing a more nuanced understanding of changes in the younger generations’ premarital sexual behaviour (PMSB). Recognising this gap, this study explores the dynamics of PMSB among AY aged 15-24 years in Nepal, a country undergoing major social, economic and cultural transitions, and where PMS is still heavily stigmatised. This study has two objectives: to investigate the social determinants of PMSB in Nepal and, in particular, the factors associated with marked gender differences in PMSB; and to centre the voices of AY themselves so as to better understand their perspectives and experiences. Using a social determinants of health (SDH) approach, mixed methods are employed in a sequential fashion. It begins with statistical analysis of secondary data from a large-scale national survey—the Nepal Adolescent and Youth Survey (NAYS) conducted in 2010/11. This analysis addresses questions relating to the social determinants of PMSB and gender differentials. The qualitative component involves interviews and focus group discussions with AY and key social agents including parents and teachers. By allowing AY to narrate their perceptions of PMS from their own complex standpoints, this part of the study addresses questions relating to the importance of context and AY subjectivities. Unsurprisingly, the NAYS analysis identified major gender differences with 23% of males reporting that they had experienced PMSB compared to just 4% of females. For both males and females, the likelihood of having PMS was most strongly associated with marital status, prior involvement in at least one ‘love affair’, having a permissive attitude towards PMS, and experience of sexual abuse. The direction of association between marital status with PMSB varied by gender. Ever-married females were 2.5 times more likely to have experienced PMS than never-married females, while never-married males were 1.5 times more likely to experience PMS than their ever-married counterparts. Unlike females, males were also more likely to have had PMS if they had got married later and undertaken some form of migration, increasing exposure time and opportunities to engage in PMS. The in-depth interviews and focus groups allowed for much richer insights into AY perspectives. The findings suggest that gender differentials in PMSB among AY in Nepal is largely due to three reasons: gendered norms and values in relation to PMS; gender differences in vulnerabilities to the consequences of PMS; and gender differences in the opportunity to experience PMS. All of those interviewed were either against PMS or for it – with little middle ground – and with significant differences by age and gender. The majority of the adult participants and female AY had negative attitudes towards PMS, whereas male AY attitudes were more diverse. The findings also suggest that there are three distinct contexts in which AY could experience PMS: within intimate love affairs, in transaction (i.e., with sex worker, or informal exchange of offer), and in coercive contexts. The use of the SDH theoretical approach in this study is somewhat novel given the dominance, in the literature, of individualistic approaches to exploring PSMB among AY. It enabled me to situate the relationships between individuals’ exposure, attitudes, and behaviours and their PMSB within the broader social context in which AY grow up, live and work. As such, this study shows that the change in PMSB among AY in Nepal is significantly influenced by changes in the social structure, increasing both opportunities and vulnerabilities (e.g., when coerced) to experience PMS. The use of a mixed methods approach also helped to surface the multidimensional realities around PMSB among AY and made visible the limitation of relying on a single method. While this study was limited in its ability to operationalise social determinants, its conceptual framework can be used to incorporate many other social determinants of PMSB at different levels, and to develop locally grounded theories that are more sensitive to local social context. Finally, this study suggests that the reporting of PMS, particularly among females, is likely to be understated due to the heavy social stigma that they have to navigate. Therefore, future studies should look to employ new methods of data collection that are gender sensitive, in order to get a more coherent and accurate picture of PMSB among AY in Nepal.