Background: TB (tuberculosis) is an infectious disease affecting millions of people worldwide; globally, an estimated 10 million people developed TB disease in the past two years; and there were an estimated 1.4 million TB deaths. In Ethiopia, the burden of TB is ranked third and seventh in Africa and the world, respectively. Hence, studies are limited to solving many of the problems related to undernutrition. Therefore this study aimed to assess the prevalence and associated factors of undernutrition among adult TB patients attending Amhara National Regional State hospitals, Northwest Ethiopia., Methods: A cross-sectional study was conducted in the TB clinics of Amhara Regional State Referral hospitals from March to April 2020. A total of 405 TB patients participated in the study. Proportional allocation and Systematic random sampling were employed to get representative study participants. The outcome variable, undernutrition, was determined by using Body Mass Index (BMI). Interviewer administered structured questionnaire, anthropometric measurements and patients' document review were done. Bivariable and multivariable logistic regressions were done to identify factors associated with undernutrition. Crude odds ratios (COR) and adjusted odds ratios (AOR) with 95% confidence intervals and P-values < 0.05 were employed to declare the significance and strength of association., Results: The prevalence of under nutrition was 42.2% [95%CI: 32.8, 51.6]. The mean (±SD) BMI was 19.30(±2.83). The odds of undernutrition was higher among TB patients with poor wealth status [AOR = 2.39, 95%CI; 1.19, 4.79], alcohol consumption [AOR = 1.57; 95%CI, 1.01, 2.47], bedridden [AOR = 3.02, 95%CI; 1.21, 7.50] and ambulatory patient [AOR = 2.11, 95%CI; 1.36, 3.26]. Furthermore, being farmer [AOR = 2.59;95%CI,1.08,6.20], housewife [AOR = 2.72;95%CI,1.22,6.06] and unemployed [AOR = 2.46;95%CI,1.18,5.13] were significantly associated with undernutrition., Conclusion and Recommendations: This study illustrated that a considerably high proportion of TB patients were undernourished in Amhara regionals state referral hospitals. Behavioral and socio-economic characteristics were significantly associated with undernutrition. Therefore, nutritional and social support activities are essential for TB patients, particularly those who are socio-economically disadvantaged. Regular nutritional screening and management, as well as behavioral interventions, should be strengthened., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2021 Published by Elsevier Ltd.)