Sub-Saharan Africa remains to be the most heavily affected region by malnutrition, accounting for 23.8% share of the global burden. Undernutrition weakens the immune system, increases the susceptibility to infections, and may worsen the impact on various kinds of diseases. Our aim was to assess undernutrition and its associated factors among AIDS-infected adult patients from Asella, Oromia Region, Ethiopia. An institutional-based cross-sectional study design was employed from June to July 2018. A total number of 519 patients were selected for the proposed work. Data was entered into EpiData, checked, coded, and analyzed using SPSS version 21 software. Descriptive statistics were used to assess the prevalence of undernutrition among patients. Bivariate and multivariate regressions were used to determine the relationship between undernutrition and its associated factors among the study participants. The results of our study showed that the overall prevalence of undernutrition was 18.3%; out of which 12.7% were mildly and 5.6% were moderately to severely undernourished, respectively. Monthly income (AOR: 3.589, 95% CI (1.469-8.768)), whole grain feeding (AOR: 2.979, 95% CI (1.252-7.088)), opportunistic infections in the last six months (AOR: 3.683, 95% CI (3.075-4.411)), clinical stage (AOR: 2.998, 95% CI (1.269-7.083)), and insufficient quality of food (AOR: 3.149, 95% CI (1.339-7.406)) were found to be significantly associated with undernutrition in this study. Therefore, HIV treatment facility should be supported with nutritional assessment, supplementation, counseling, care, and support to patients that may possibly alleviate this predicament.