Background: Preterm birth is defined as a delivery that occurs at less than 37 weeks of gestation. The majority of preterm birth remains vulnerable to long term complications that may persist all over their lives. Globally, about 12.9 million births (9.6%) of all births worldwide were preterm and of these more than 60% of preterm births occur in Africa and South Asia while about 0.5 million were in each of Europe and North America. There is limited evidence on the magnitude of preterm birth and associated factors among women attending delivery service at public hospitals of low-income countries like Ethiopia including the study setting, Harar town. Objective: To assess the magnitude of preterm birth and associated factors among mothers who gave birth in public health hospitals in Harar town, Eastern Ethiopia, 2019. Methods and material: An institutional-based cross-sectional study was conducted on 325 women attending delivery service in Harar public hospitals. Structured questionnaires were used to collect data; a systematic sampling technique was also used to select participants. A total of three data collectors and one supervisor participated in the study. The data was entered into SPSS version 22.0 for analysis. Pretest, double data entry and local language translation were used to assure data quality. Descriptive and logistic analysis was employed. To measure the strength of association between dependent and independent variables, Crude and Adjusted Odd Ratios with 95% confidence interval were calculated. Finally, the variable which shows p-value<0.05 considered as statistically significant. Result: The study showed that the magnitude of preterm birth was 24.9% (95%CI 21.0, 29.8). Women who didn’t attend ANC (AOR=1.5, 95% CI: 0.7, 2.3), with a history of antepartum hemorrhage (AOR=1.3, 95% CI: 0.2, 2.7), hemoglobin less than eleven (AOR=1.4, 95% CI: 0.2, 2.2), birth interval less than 24 months (AOR=1.3, 95% CI: 0.4, 2.3), and history of chronic disease (AOR=1.3, 95% CI: 0.4, 2.3) were significantly associated with the outcome variable. Conclusion and recommendation: The prevalence of preterm birth in Harar town public health hospitals is slightly higher than studies done in different parts of Ethiopia. Not attending ANC, short interpregnancy interval (<24 months), previous history of APH, presence of chronic medical illness, and low hemoglobin level (<11g/dl) were found to be statistically significant with the occurrence of preterm birth in the current pregnancy, therefore, efforts are needed to improve it.