Background: Both Human Immunodeficiency Virus (HIV) and malnutrition can independently cause progressive damage to the immune system. The former increases susceptibility to infection, morbidity and mortality through opportunistic infections. Malnutrition hastens progression to AIDS related illnesses; undermines adherence and response to antiretroviral therapy (ART) in resource-poor settings. Objectives: To asses under nutrition and associated factors among adults living with HIV/AIDS in Addis Ababa. Methods: An Institution based cross- sectional study was conducted on March 2019 and simple random sampling technique was used to select study subjects with a total of clients 504 were included in the study. Weight and height of the patient was measured to estimate the magnitude of under nutrition in addition of face to face interview. The study subject was interviewed based on the questionnaire. The degree of association was assessed using odds ratio with 95% confidence interval and P value. Level of statistical significance was declared at p value less than 0.05. Result: The prevalence of under nutrition in this study was 23.8 %( 95% CI 12.8-24.8). In the multivariable logistic regression analysis under nutrition (BMI<18.5) was significantly associated with those who are divorced (AOR=4.82, 95%CI (1.09-21.55)), being Anemic (AOR= 3.02, 95%CI (1.42-6.43)),WHO clinical stage I&II (AOR=.03,95%CI(.01-.09)and AOR=.19,95%CI(.07-.51)respectively) and clients with poor food consumption score(AOR=10.07,95%CI(3.12-32.5)). Conclusion: There is a burden of under nutrition in people living with HIV/AIDS. Being divorced, presence of anemia, current ART regimen, late WHO T- clinical stage and poor food consumption score were associated with under nutrition in this study.