Pre-exposure prophylaxis (PrEP) is an HIV-prevention strategy recommended for those at high-risk of infection, including adolescents and young people (AYP). We explored how PrEP roll-out could influence sexual risk behaviour among AYP in East and southern Africa. Twenty-four group discussions and 60 in-depth interviews were conducted with AYP between 13 and 24 years old, recruited from community settings in Uganda, Zimbabwe and South Africa, from September 2018 to January 2019. Participants perceived that PrEP availability could change sexual behaviour among AYP, influencing: (1) condom use (increased preference for condomless sex, reduced need and decrease in use of condoms, relief from condom use discomfort, consistent condom use to curb sexually transmitted infections and pregnancies); (2) sexual activities (increase in sexual partners and sexual encounters, early sexual debut, sexual experimentation and peace of mind during risky sex, sexual violence and perversion); (3) HIV risk perception (neglect of other HIV prevention strategies, unknown sexual partner HIV status, adoption of PrEP). PrEP initiation may be associated with increased interest in sexual activities and risky sexual behaviour among AYP. PrEP should be included as part of a combination package of HIV prevention strategies for AYP with methods to prevent other sexually transmitted infections and unwanted pregnancies.