Even though antiretroviral treatment is becoming more efficient and available, new HIV infections still occur. This is particularly the case in sub-Saharan Africa. Sexual transmission of HIV is still the main mode of transmission in sub-Saharan Africa, and multiple sex partners have been shown to be crucial for the spread of the epidemic. It is therefore problematic that sexual risk-taking, in terms of multiple sex partners, persists in spite of HIV awareness and knowledge. This study examines the role of social norms on multiple partnerships using longitudinal data on young adults residing in the metropolitan communities of Cape Town, in South Africa. Overall, our results show that social norms, proxied by the average number of sex partners in the community, have a positive and significant influence on young adults’ choice of number of sex partners. This effect appears to be stronger amongst male young adults, than female young adults.