Background: Adherence to antiretroviral therapy (ART) is a key factor in managing HIV/AIDS. Positive beliefs about ART among people living with HIV (PLHIV) are crucial for maintaining good treatment adherence, whereas negative beliefs about ART result in low treatment adherence. Objective: This study aimed to analyze the association between beliefs regarding treatment and ART adherence among PLHIV receiving outpatient care. Methods: A cross-sectional study was conducted on 157 PLHIV at the HIV clinic of Universitas Airlangga Hospital between April and June 2025. Adherence was measured using the Self-Reported Adherence (SERAD) questionnaire and pill count, with a cutoff of ≥80%. The relationship was analyzed using binary logistic regression analysis. Results: Most respondents held positive beliefs regarding the necessity of ART (87.9%) and were classified as adherent (86%). However, no statistically significant associations were observed between ART adherence and the BMQ domain scores for necessity, concerns, overuse, or harm. The odds ratios were 1.00 (95% CI: 0.88–1.12) for necessity, 0.99 (95% CI: 0.89–1.12) for concerns, 0.97 (95% CI: 0.82–1.16) for overuse, and 0.97 (95% CI: 0.82–1.15) for harm. Psychological factors such as depression and low motivation, as well as practical reasons such as forgetfulness, were identified as causes of non-adherence despite a strong belief in the medication. Conclusion: Although most PLHIV had strong beliefs and adherence, no statistically significant association was observed between beliefs about treatment and ART adherence. These findings suggest that ART adherence may be influenced by multiple factors beyond treatment beliefs, underscoring the need for holistic and comprehensive interventions that address the diverse barriers to sustaining ART adherence.