Background: Undernutrition and active TB infections are interrelated and exacerbate one another in various ways. Active TB infections can lead to undernutrition while undernutrition can in turn lead to greater prevalence and severity of tuberculosis. There are a number of adult tuberculosis patients suffering from undernutrition in the world especially in developing countries. One-third of the world’s population and 50% of adults in sub Saharan Africa, and South-East Asia, are infected individuals at risk for developing tuberculosis. Objective: The main aim of the study to assess the prevalence of undernutrition and associated factors among adult TB patients in public health Institutions in Bahir Dar town, North West Ethiopia, 2017. Method: Institution based cross sectional study was conducted among adult TB patients in public health facility in BahirDar town from February to March 2017.The study was conducted among 368 participants were selected using systematic random sampling. A pre-tested structured questionnaire was used to collect the data. Data was entered epi-info version 3.1.5 and analyzed using SPSS version 20 by the investigator. Independent variables with P-value less than 0.20 in the bivariate analysis were included in the multivariable logistic regression model to identify factors associated with the dependent variable. Result: The prevalence of undernutrition was 43.5 %.Out of this the patterns were 29.9% mild (17.0-18.49kg/m2), 11.1% moderate (16.0-16.99kg/m2) and 2.4% severe (<16.0kg/m2) undernutrition. Patients who had not received dietary counseling were 3.9 times more likely to be undernourished compared with those who had received dietary counselling [AOR= 3.95, 95% CI :2.25, 6.60]. Patients with ambulatory functional status were more likely to be malnourished compared to those who had working functional status [AOR=2.25, 95%CI: 1.29, 3.94].There was a statistically significant association between monthly income and undernutrition [AOR=5.11, 95% CI: 1.32, 19.82]. Conclusion and Recommendations: The prevalence of undernutrition among adult TB patients in the study is very high. Dietary counseling, functional status of the patients and monthly income were significantly associated factors of undernutrition. Dietary counseling can have a substantial role in providing the recommended intake of nutrients and improving the nutritional status of tuberculosis patients.