Undernutrition is a major public health problem worldwide, particularly in developing countries like Ethiopia. However, nutritional problems are frequently overlooked in low-income countries, especially among vulnerable populations such as imprisoned people. The scientific data on the rate of undernutrition among imprisoned people in Ethiopia is limited. Hence, this study aimed to assess the magnitude and associated factors of undernutrition among adult prisoners in Fiche town, central Ethiopia. A facility-based cross-sectional study was conducted from August 15 to September 15, 2020. A systematic random sampling technique was used to select participants. All prisoners whose age was 18 years and above who have been in prison for at least 6 months were included. Data were collected using interviewer-administered pretested semi-structured questionnaires and standard anthropometric measurements. A cut-off point of body mass index <18.5 kg/m2 was used to measure undernutrition. Data were coded, entered into Epi-data version 3.1, and analyzed using Statistical Package for Social Sciences version 20.0. A binary logistic regression analysis was conducted to identify factors associated with undernutrition. The adjusted odds ratio (AOR) with a 95% confidence interval (CI) was calculated to measure the strength of the association and a p-value of less than 0.05 was considered statistically significant. The overall magnitude of undernutrition among adult prisoners was 20% (95% CI: 16.5-23.6). Duration of imprisonment, incarcerated for 25 to 59 months (AOR = 3.07; 95% CI: 1.33, 7.04) and for greater than 59 months (AOR = 4.56; 95% CI: 2.0, 10.45), mild and moderate depression (AOR = 1.9; 95% CI: 1.05, 3.45), and moderately severe and severe depression (AOR = 2.78; 95% CI: 1.17, 6.60) were significantly associated with increased odds of undernutrition. However, being female (AOR = 0.51; 95% CI: 0.26, 0.98), having financial support (AOR = 0.36; 95% CI: 0.15, 0.87), engaging in income-generating work within the prison (AOR = 0.27; 95% CI: 0.15, 0.47), having medium dietary diversity (AOR = 0.35; 95% CI: 0.15, 0.80), and having good dietary diversity (AOR = 0.23; 95% CI: 0.08, 0.61) significantly decreased the odds of undernutrition. The magnitude of undernutrition among adult prisoners was high, with one in five prisoners in Fiche town prison having undernutrition. Sex, financial support, duration of imprisonment, income-generating work in the prison, dietary diversity, and depression were predictors of undernutrition. Hence, access to healthy food and diversified diets should be ensured for prisoners, and implementing early screening and treatment of depression, as well as encouraging prisoners to engage in income-generating work within the prison is recommended to reduce the burden of undernutrition.