Background: Tuberculosis is a chronic contagious disease affecting millions of people globally. An estimated 1.3 million people died because of tuberculosis and about 10.4 million new tuberculosis cases reported in 2016. Tuberculosis remains one of the major public health problems among different communicable diseases in Ethiopia. Nevertheless, little is known about magnitude of undernutrition and associated factors among tuberculosis patients in low income countries like Ethiopia. Objective: This study was aimed to assess magnitude of undernutrition and associated factors among adult tuberculosis patients in public health facilities of Dire Dawa administration and Harari region, Eastern Ethiopia. Methods: Institutional based cross-sectional study design was conducted among 452 randomly selected adult tuberculosis patients at public health facilities of Dire Dawa administration and Harari region from April 4, 2018 to May 24, 2018. Interviewer administered questionnaires, tuberculosis treatment registration book review and anthropometric measurements were used to collect data. Data were entered to Epidata version 3.1 and exported to SPSS version 20.0 for analysis. Bivariable and multivariable analyses were done to identify independent factors associated with undernutrition. Level of statistical significance was declared at P-value <0.05. Results: The magnitude of undernutrition (BMI<18.5kg/m2) was found to be 50.2%; 95% CI (45.8, 54.9). Having eating problem [AOR=2.10, 95% CI (1.314, 3.317)], secondary educational status [AOR=2.81, 95% CI (1.118, 7.066)], married marital status [AOR=0.55, 95% CI: (0.301, 0.990)] and PTB –ve [AOR=2.92, 95% CI: (1.016, 3.637)] were significantly associated with undernutrition. Conclusion: The magnitude of undernutrition was high in the study area. Having eating problem, being married, PTB-ve TB type and having secondary educational status were significantly associated with undernutrition. Hence, collaborative and timely actions by different concerned bodies need to be taken to improve the nutritional status of tuberculosis patients considering the factors which have association with undernutrition