Background <p>Viral load is the most reliable indicator of antiretroviral therapy response and treatment failure. In Ethiopia, virologic failure remains a major challenge despite expanded ART coverage.</p> Objective <p>To identify determinants of virologic failure among adults on first-line antiretroviral therapy in health facilities of Awi Zone, Northwest Ethiopia.</p> Methods <p>An institution-based unmatched case–control study with a 1:2 ratio was conducted among 369 participants (123 cases and 246 controls). Cases were HIV-infected clients aged ≥ 15 years with virologic failure (viral load &gt; 1000 copies/mL on two consecutive tests after adherence support) between 30 October 2020 and 30 December 2024. Controls were HIV-infected clients aged ≥ 15 years on first-line ART with viral load ≤ 1000 copies/mL on two consecutive tests after ≥ 6 months of treatment during the study period. Participants were selected using computer-generated simple random sampling from selected facilities. Bivariable and multivariable binary logistic regression analyses were performed to identify independent determinants of virologic failure.</p> Results <p>Multivariable analysis identified non-disclosure of HIV status (AOR = 2.24, 95% CI: 1.11–5.22), viral load after ART initiation &gt; 1000 copies/mL (AOR = 5.32, 95% CI: 3.32–7.15), poor treatment adherence (AOR = 5.64, 95% CI: 2.01–7.32), treatment interruption (AOR = 6.45, 95% CI: 3.95–14.01), being single (AOR = 2.34, 95% CI: 1.54–5.32), and CD4 count &lt; 200 cells/mm³ (AOR = 4.90, 95% CI: 2.40–8.25) as significant determinants of virologic failure.</p> Conclusion <p>Strengthening adherence support, retention in care, and disclosure counseling is essential to reduce virologic failure.</p>