Background: Preterm delivery is a major global health concern that continues to be a primary focus in obstetric and pediatric units in Sub-Saharan African countries. Preterm birth complications are the largest cause of mortality in children under the age of five worldwide and higher in sub-Saharan Africa. The aim of this study was to investigate the magnitude of preterm delivery and its associated factors at Tanzania’s Muhimbili National Hospital, a tertiary hospital. Methodology: An institutional based cross-sectional study was undertaken at Muhimbili National Hospital’s maternity ward in the year 2021.The study enrolled 422 mothers who met the criteria. Mothers were interviewed face to face using a structured questionnaire, other clinical data were collected from medical records and the registry. Chi square (x2) and Fisher exact tests were used to evaluate the associated factors. The strength of association between the independent and dependent variables was determined using a logistic regression model. Results: In this stu the proportion of mothers who had preterm delivery was 19.91%. Previous preterm birth (aOR=5.4, 95% CI 2.53-11.34), short Interpregnancy interval (aOR=3.0, 95% CI 3.08-9.03), maternal ANC visits less than 4 contacts (aOR = 3.0, 95% CI 1.79-8.38), pregnancy-related hypertension (aOR = 10.8, 95% CI 5.75, 18.33), antepartum hemorrhage (aOR = 14.5, 95% CI 4.92-24.27), preterm prelabor rupture of membranes (aOR =5.9, 95% CI 1.94-14.12), multiple pregnancy (aOR =10.7, 95% CI ,10.21-92.69) and maternal urinary tract infection (aOR =3.1, 95% CI,1.94-8.15) found to be independent factors associated with preterm deliver