Background: Almost 1.9 billion adults worldwide are overweight or obese, while 462 million are underweight. These are primarily found in countries with low and middle-incomes, like Ethiopia. Under nutrition is the frequent health problem among peoples living with HIV/AIDS, however no large-scale research, including a number of health facilities has been conducted in Ethiopia. So this study aimed to assess the nutritional status and under nutrition related factors among HIV/AIDS patients attending ART follow-up in public hospitals of Bench Sheko zones, south west, Ethiopia.Methods: In November/2020, a cross-sectional facility study design was carried out in all public hospitals in the Bench Sheko zone. To pick the study participants, a systematic sampling technique was used. Data were collected via face-to-face interviews and entered into Epi-data 3.1, then exported to SPSS version 24 for statistical analysis. In order to identify the candidate and independent determinants of under-nutrition, bivariate and multivariable logistic regression analyses were carried out respectively. Independent determinants of under-nutrition were measured using adjusted odds ratio with their corresponding 95% confidence level at P-value of < 0.05 cut of point.Results: A total of four hundred two adult HIV-positive patients who were attending in ART clinics were participated with 98% response rate. The proportion of under nutrition (BMI<18.5 kg/m2), normal nutrition (BMI 18.5–24.9 kg/m2) and overweight or obese (BMI ≥25 kg/m2) patients were 29.2% [95% CI: (24.6-33.5)], 61.2% [95% CI: (56.5-66.3)], and 9.6% [95% CI: (6.6-12.9)] respectively. Out of under nutrition patients, severe malnutrition (BMI<16 kg/m2) accounted of 5.6%. Factors food insecurity [AOR: 3.26, 95% CI: (1.79-5.92)], poor ART adherence [AOR: 4.22, 95% CI: (1.47-12.14)], CD4 cell count ≤ 200 [AOR: 5.73, 95% CI: (2.45-13.13)], and substance user [AOR: 4.11, 95% CI: (2.31-7.30)] was independent factors of under nutrition.Conclusion: This study found that the prevalence of malnutrition was high compared to other settings. The government should also pay due attention to improving the treatment of HIV/AIDS by offering nutritional support services in hospitals. Moreover policymaker and heath care professionals consider the effects of these factors for nutrition while providing ART service.