Background: Globally about 15 million newborns are born preterm every year. Over one million of them die due to complications related to prematurity. Preterm birth was a leading cause of neonatal death in Ethiopia. One in four of neonatal deaths in Ethiopia were due to prematurity related complications. But studies to identify determinants of preterm birth in the study area are limited. Objective: To identify determinants of preterm birth among newborns delivered in Bahir Dar city public hospitals, North West Ethiopia in 2019. Method: An institutional based unmatched case control study was conducted in 314 samples with 105 cases and 209 controls among newborns delivered in Bahir Dar city public hospitals. The sample size was calculated by Epinfo version7. All individual cases were selected consecutively. For each cases two controls were selected by using systematic random sampling technique. Data was collected using interviewer administrated structured questioners. The collected data was entered into Epinfo version 7 and then exported to SPSS version 20. Independent variables with p-value less than 0.2 in the bi-variable analysis were entered into multivariable logistic regression model. Statistical significance level was declared at p-value less than 0.05. Model assumption were checked by Hosmer-lemeshow goodness-of-fit test. Result: The study identified; low birth interval AOR = 2.28 (95 % CI= 1.18- 4.42), lower number of ANC visit AOR = 3.89 (95 % CI =2.17- 6.97), previous history of preterm birth AOR = 5.69 (95 % CI =1.93- 16.66), premature rapture of membrane AOR = 3.58 (95 % CI =1.11-11.54), preeclampsia/eclampsia AOR = 2.86 (95 % CI =1.01- 8.08), lower level hemoglobin level AOR = 3.89 (95 % CI =2.01- 7.51) were positively associated with preterm birth. Conclusion: Previous history of preterm births, preeclampsia and premature rapture of membrane, fewer number of antenatal care visit, lower birth interval, and lower hemoglobin level were found determinants of preterm birth. Therefore encouraging women to attend four or more antenatal care visits, health education on the importance of birth spacing and anemia prevention and treatment may help to prevent preterm birth.