Background. Preterm birth infants are at a greater risk of mortality and a variety of health and developmental problems; reliable data support that this rate is increasing in almost all countries. The purpose is to find the magnitude of preterm birth and its associated factors among newborns delivered at Butajira Hospital, Southern Nations, Nationalities, and People’s Region, Ethiopia. Methods. This hospital-based cross-sectional study was carried out on 304 maternity cards using the systematic sampling method during May 1_21 in 2019. The data collection was performed using a structured case record form. The data were entered into EpiData software (version 3.1) and analyzed using SPSS software (version 21). Binary and multivariable logistic regression analyses were computed to identify the associated factors at 95% CI. Results. Overall magnitude of preterm birth was observed to be 15.5%. The factors significantly associated with the preterm birth were preeclampsia (AOR=4.0; 95% CI: 1.6-10.0), and premature rupture of fetal membranes (AOR=4.2; 95% CI: 1.4-12.9). Conclusion. Preterm birth is still public concern in the study area. The concerned administrative body should implement health education programs and improve the quality of health care delivered to pregnant mothers to control these associated factors and, consequently, promote public health in the study area.