Background Tuberculosis is one of the top ten causes of illness, death, and disability throughout the world. Undernutrition reduces immunity, which makes latent tuberculosis more likely to become active tuberculosis. Tuberculosis makes these conditions worse. The body of a person suffering from TB has an increased demand for energy, which often causes a TB patient to lose a significant amount of weight and this can worsen acute undernutrition. The aim of this study was to assess the magnitude of undernutrition and its associated factors among adult TB patients in public health facilities in Haramaya district, eastern Ethiopia. Methods Institution-based cross-sectional study was conducted among 330 adult tuberculosis patients on follow-up in public health facility of Haramaya District, eastern Ethiopia from January 10, 2021 to February 20, 2021. An anthropometric assessment was done after a face-to-face interview using a pretested structured questionnaire. SPSS 24 was used to analyze the data. Bivariable and multivariable logistic regression model was used to identify factors associated with undernutrition. Results The overall prevalence of undernutrition was 43.6% (95% CI 38.2–49.1%). Proportion of severe, moderate and mild undernutrition was 11.8%, 12.4%, and 19.4%, respectively. Age group of 18–24 years (AOR = 4.12; 95% CI 1.36–12.51), not have formal education (AOR = 1.76; 95% CI 1.01–3.08), having large family size (AOR = 2.62; 95% CI 1.43–4.82), low dietary diversity (AOR = 2.96; 95% CI 1.75–4.99), lack of latrine (AOR = 2.14; 95% CI 1.26–3.65), history of TB treatment (AOR = 2.56; 95% CI 1.19–5.54) and taking intensive phase of anti-TB drugs (AOR = 3.18; 95% CI 1.62–6.25) were factors found significantly associated with under nutrition. Conclusion The prevalence of undernutrition was high. Age, educational status, family size, dietary diversity, toilet facility, history of tuberculosis medication and intensive phase of anti-TB drugs were found significantly associated with undernutrition. The nutritional derangement could call for fast nutritional intervention in the management of pulmonary tuberculosis patients.