Young people living with perinatally acquired HIV may be at risk of poor adherence to antiretroviral therapy; identification of predictors, using a conceptual framework approach proposed previously by others, is important to identify those at higher risk. In 261 young people with perinatally acquired HIV in England, 70 (27%) reported 3-day nonadherence, 82 (31%) last month nonadherence, and 106 (41%) nonadherence on either measure. Of those reporting nonadherence on both measures, 52% (23/44) had viral load of <50 copies/ml, compared with 88% (127/145) of those reported being fully adherent. In multivariable analysis, young person and medication theme factors were associated with nonadherence. The main predictors of 3-day nonadherence were antiretroviral therapy containing a boosted protease inhibitor and poorer quality of life. Predictors of last month nonadherence were having told more people about one's HIV status, worse self-perception about having HIV, and boosted protease inhibitor-based regimens. The consistency of individual young person and medication factors in predicting nonadherence gives insight into where interventions may best be targeted to improve adherence.