Globally, every year, 1.1 million newborns die due to prematurity. In Ethiopia, 320,000 preterm births occur each year; out of these, 24,400 deaths were due to preterm complications. However, there is little evidence about preterm birth in the study area. Therefore, this study provides an important direction for health professionals, health programmers, and researchers. A facility-based unmatched case-control study design was employed among 244 women (61 cases and 183 controls) who gave birth in Dilla University Referral Hospital and were selected with purposive sampling. The bivariate and multivariable logistic regression model was used to select independent predictors of preterm birth. The multivariate analysis was used, and the results were interpreted using an adjusted odds ratio at 95% confidence interval and statistically significant level at a P value less than 0.05. A total of 240 mothers (60 cases and 180 controls) were included in the study with a 98.3% response rate. Factors like attending secondary educational and above [adjusted odd ratio aOR = 0.07 (0.08-0.65)] and attending antenatal care [ aOR = 0.41 (0.18-0.93)] were protective whereas having urinary tract infection [ aOR = 3.6 (1.1-11)], having human immune virus diseases [ aOR = 4.2 (0.9-18)], having a history of abortion [ aOR = 2.3 (1.1-5)], having a history of preterm delivery [ aOR = 5 (1.6-15)], and having hypertensive disorders of pregnancy [ aOR = 5 (1.9-13)] were significantly associated risk factors for preterm birth. The main determinant factors for preterm birth are having antenatal care follow-up, attending secondary education and above, hypertensive disorders of pregnancy, having HIV/AIDS, and history of abortion. This shows a need to strengthen female education; screen mothers for HIV/AIDS, urinary tract infection, and hypertension; and strengthen nutritional counseling, during ANC visits.