Male circumcision has both health benefits and significance to some cultures. We sought to understand perceptions about male circumcision as part of the HIV prevention toolkit among participants enrolled in a preventive HIV vaccine efficacy trial in South Africa. We conducted a qualitative study with 28 people aged 18-35 years old who self-reported that they were not living with HIV, provided informed consent, and who were participating in the HVTN 702 vaccine efficacy trial in Soweto. Using a semi-structured guide, we facilitated four focus group discussions (FGDs) stratified by age, gender, and sexual orientation. FGDs were audio-recorded, transcribed verbatim, translated into English, and thematically analysed. Four main themes emerged. (1) Everyone had accurate knowledge about what male circumcision is, and some participants stated that it partially reduces acquisition of HIV and sexually transmitted infections. (2) There was an emerging distrust of cultural circumcision because of perceived lack of transparency and adverse events. (3) There was a perception that circumcision boosted masculinity. (4) The choice to circumcise was influenced by parents, family, and female partners. In conclusion, the study found that young South African HIV vaccine trial participants accurately understand the HIV prevention benefits of male circumcision, but uptake decisions are embedded within a context that is informed by culture, sexuality and masculinity norms and values.