Public health problems related to tuberculosis (TB) remain substantial globally, particularly in resource-limited countries. Determining TB treatment outcomes and identifying contributing factors are the basic components of the TB control strategy. In Ethiopia, different studies have been done on treatment outcomes and multiple associated factors, and there is also a little information on the effect of nutritional status on TB treatment outcomes. So there is a need for comprehensive research that examines the combined effects of multiple factors along with nutritional status. A five-year institution-based retrospective cross-sectional study was conducted at Mizan Tepi University Teaching Hospital, South West Ethiopia. This study included all tuberculosis patients who were documented in the TB registration and had known treatment outcomes at the treatment facility between January 1, 2016, and December 31, 2020. Data was collected through a pretested structured data extraction checklist. Data were entered into Epidata version 3.1 and analyzed through SPSS version 22. Multiple logistic regression was employed to assess the association between dependent and independent variables. A p-value of less than 0.05 was considered statistically significant. Of the total 625 TB patients, 283 (45.3%), 175 (28%), and 167 (26.7%) had smear-positive, extra-pulmonary, and smear-negative tuberculosis, respectively. The majority of study participants had normal weight (62.2%), were in the age group of 15-44 (67.4%), were new cases (73.8%), and were from urban areas (69.4%). About 32.2% of cases were HIV-positive. The overall unsuccessful treatment rate was 25%. From the total unsuccessful treatment rates, the highest proportion was a death rate of 90 (14.4%), followed by a treatment failure of 56 (9%). Being female (AOR = 1.7, 95% CI: 1.2-2.5), HIV positive (AOR = 2.7, 95% CI: 1.9-4.1), undernutrition (BMI<18.5kg/m2) (AOR = 1.9, 95% CI: 1.3-2.9), and smear-negative pulmonary TB (AOR = 1.6, 95% CI: 1-2.5) were independent predictors of unsuccessful treatment outcomes. The treatment success rate in the study area is very poor. Poor treatment outcomes were associated with undernutrition, female gender, HIV positivity and smear-negative pulmonary TB. So, continuous and serious supervision and monitoring of directly observed treatment short course (DOTS) program accomplishment, early detection of HIV and TB, prompt anti TB and antiretroviral treatment initiation and adherence, enhanced nutritional assessment, and counseling services need to be strengthened to improve treatment outcomes.