Introduction; Undernutrition denotes insufficient intake of energy and nutrients to meet an individual's needs to maintain good health. Undernutrition is a common problem among peoples living with HIV, which creates a vicious cycle that may catalyze progression from HIV infection to AIDS .Undernutrition, continues a major public health burden Worldwide, mainly in resource constraint countries. The condition has no exception in Ethiopia where undernutrition is most prevalent. Thus, generating updated evidence might be helpful to HIV patients and the health system to enhance the health conditions of HIV patients. Objective: To assess the prevalence and associated factors of undernutrition among adult on antiretroviral therapy in Bure town health institution. Method: -An Institution based cross-sectional study was conducted among 301 adults on Anti-retroviral therapy from Feb 1/2020- Apr. 1/ 2020. Systematic sampling method was used to get the study subjects. Weight and height measurements were taken using standardized weighting scales and measuring boards respectively. Data were-were collected by trained nurses using a pretested interviewer- administered questionnaire. Datawere entered and analyzed using SPSS version 25. Bivariable and multivariable logistic regression analysis wasapplied to identify factors associated with undernutrition among ART patients. A p-value <0.05 was used to determine statistical significance. Results: The prevalence of under nutrition (body mass index [BMI] ≤18.5 kg/m2) among patients on ART was 78(25.9%). Under nutrition was significantly associated with Adherence to HAART (AOR) = 8.57, 95% (CI: 3.31, 22.18,) CD4 count (200-500) (AOR =0.08, 95% CI: 0.02-0.28) and CD4 >500 (AOR = 0.04, 95% CI: 0.01, 0.19)), and food insecurity (AOR = 4.84, 95% CI: 2.5, 9.55) Conclusion:The prevalence of under nutrition was high among adult people on ART despite of extensive usage of HAART in the study area. Predictors of undernutrition were adherence to HARRT, CD4 cell and food insecurity. Further, intervention initiatives should focus on improving adherence to highly active antiretroviral therapy (HAART), enhancing CD4 count household food security in adult HIV infected individuals to address the problem of under nutrition.