Highly Active Antiretroviral Therapy (HAART) has substantially declined morbidity and mortality related to Human immunodeficiency virus/Acquired immunodeficiency syndrome (HIV/AIDS). Despite this fact, first-line ART failure has emerged as a growing concern. However, factors associated with first-line ART failure are not well empathized and studied. Hence, we aimed to identify the determinants of first-line ART failure among patients attending ART in Public Hospitals Jimma, Southwest Ethiopia. A case-control study was conducted in March 2018 on a sample of 384 (288 controls and 96 cases) adult people living with HIV/AIDS (PLWHA). Cases were HIV patients aged 15 years or older who were on first-line ART regimens with documented therapeutic failure. Controls were HIV patients aged 15 years or older who were on first-line ART regimen but without evidence of therapeutic failure. Data were extracted from electronic databases and supplemented by data collected through interviewer-administered questionnaires. Bivariate and multivariate logistic regression analyses were used. Adjusted odds ratios and 95% confidence intervals were used to report independently associated factors at P-value<0.05. In this study, higher odds of first-line ART failure was experienced among urban residents (AOR: