Background:- Adequate food and nutrtion is a central component of life in correctional institutions. Prisones usually comprise marginalized sections of society and they are risk groups nutritionaly due to inadequate diet intake, interims of quality and quantity. However, there is no adequate studies on the nutritional statues of prisons in Ethiopia including the study area. Objective:- To assess undernutrition and associated factors among prisoners in Bahir Dar Zone prison center, Ethiopia, 2021 GC. Methods:- An institution based cross-sectional study was conducted among randomly selected 582 prisoners from october 1 to oct 30,2021. Interviewer-administered sumi-structured questionnaire was used to collect the required data. Body mass index(BMI) was assessed to determine the nutritional status of prisoners. Data were entered to Epi-info statistical soft ware Version 7 and exported to SPSS version 23 for analysis. Both bi-variable and multi-variable logistic regression analysis were used to identify factors associated with undernutrition. In multi-variable binary logistic regression analysis variables with p value <0.05 were considered as significant. Adjusted odds ratio (AOR) and 95% confidence intervals (CI) were used to measure the strength of association. Results:-The prevalence of undernutrition was 17.5%(95%CI:14.3, 21). Imprisonment history(AOR:4.98,95%CI:2.80,8.86), history of cigarret smoking (AOR:5.38,95%CI:2.86,10.13), imprisonmentduration(AOR:1.82,95%CI:1.04,3.19),diarrhealdiseases(AOR:1.98,95%CI:1.15,3.41),depression(AOR:3.23,95%CI:1.88,5.57), socialsupport (AOR:7.09,95%CI:3.57,14.05) were factors significantly increases the odd of undernutrition. Conclusion: The magnitude of undernutrition was found to be 17.5% . Imprisonment history,duration of imprisonment, history of cigarret smoking,depression, social support and experiencing of diarrheal diseases were factors associated with undernutrition. Thus,appropriate social support and depression reduction intervention are essential to reduce undernutrition. Key words:- Undernutrition, Prison, Bahir Dar prison center, Ethiopia