Medication non-adherence can be intentional or unintentional. We investigated the prevalence of unintentional and intentional non-adherence to antiretroviral therapy (ART) and the relationship with beliefs about medicines, sociodemographic- and HIV-related variables among people with HIV (PWH) attending the HIV clinic of the Amsterdam University Medical Centers. Participants completed the Medication Adherence Rating Scale (MARS) and the Beliefs about Medicines (BMQ) questionnaire. About half of 80 participants reported unintentional non-adherence and 20% reported intentional non-adherence. Both unintentional and intentional non-adherence were associated with younger age. Additionally, intentional non-adherence was associated with being a migrant from Suriname /Netherlands Antilles, having more concerns about negative effects of ART and stronger beliefs that medicines in general are overused/ overprescribed. In conclusion, intentional but not unintentional non-adherence was associated with beliefs about medicines. Eliciting and discussing beliefs about medicines may be a promising avenue to address patients’ concerns and perceptions thereby potentially enhancing medication adherence.