Background: HIV/AIDS and malnutrition effects are interrelated and exacerbate one another in a vicious cycle. HIV specifically affects nutritional status by increasing energy requirements, reducing food intake and adversely affecting nutrient absorption and metabolism. In spite of the number of People Living with HIV ever enrolled on ART increases significantly in Ethiopia. Nutritional care and other supports which help for the success of treatment received insufficient attention. The aim of this study was to assess the prevalence of undernutrition status and its determinants among adult HIV/AIDS Clients enrolled on ART at Nigest Elleni Mohammed Memorial Hospital in Hosanna Town, Southern Ethiopia.Method: Institution based cross-sectional quantitative study was conducted using systematic random sampling technique with sample size of 234. From each sampled patient, interview and anthropometric data were collected. Bivariate and multivariate logistic regression analysis was used and the variables which had significant association were identified on the basis of p-value ≤ 0.05 and AOR, with 95% CI.Results: Among adult HIV/AIDS Clients in rolled in ART, 32.5% of them were under nourished (BMI<18.5 Kg/m 2 ). ART patients had no nutritional supports (AOR=2.22), patients who feed less than 3times in a day (AOR=3.29) and had smoking habit (AOR=6.06) were more likely to be under nourished and those patients in WHO Clinical stage 3 (AOR=0.12) were less likely to be under nourished. Conclusion:This study revealed that under nutrition among adult HIV/AIDS Clients in rolled in ART was high prevalent problem in the study area. WHO clinical stage, daily food intake, nutritional support, smoking status was identified as the determinants of under nutrition. Current study identified that there is a need to design and implement nutritional interventions including nutritional support in items or financially together with healthy habit counselling as part of integrated ART service for effective patient treatment outcome.