This study aimed to assess nutritional status and associated factors among adult tuberculosis patients in public health centres in Horro Guduru Wollega Zone, Western Ethiopia, 2021. An institutional-based cross-sectional study was conducted among 334 randomly selected adult TB patients at public health centres from May 7, 2021, to June 21, 2021. Data were collected using structured questionnaires and anthropometric measurements. The nutritional status was measured by using body mass index (BMI). Data was entered into EpiData version 4.6 and exported to SPSS version 25 for analysis. A bivariate and multivariable multinomial logistic regression analysis was done to identify factors associated with nutritional status. The prevalence of under and overnutrition was found to be 48.2% and 8.7%, respectively. Female TB patients (AOR = 3.48, 95% CI: (1.918–6.314)), patients who didn’t receive dietary counselling (AOR = 2.51, 95% CI: (1.335–4.720)), TB patients on the initiation phase of treatment (AOR = 3.76, 95% CI: (2.072–6.852)), and meal frequency less than three times per day (AOR = 3.6, 95% CI: (1.942–6.676)) were significantly associated with under nutrition. The prevalence of undernutrition was high in the study area. Being a female, being in the initiation phase of treatment, lack of dietary counselling, and having meal less than three per day were independently associated with undernutrition. Hence, regular nutritional assessments, dietary counselling, and nutritional support should be encouraged at the facility and community level.