Background: Preterm birth is a birth that occurs before 37 weeks of gestational age since the first day of a woman’s last menstrual period. In Ethiopia, 10% of babies born preterm each year. Preterm birth is the major cause of neonatal deaths next to pneumonia and it is a substantial cause of long-term problems in survivors. The objective of the study was to identify key determinants of preterm birth in Kembata Tembaro, Southern Ethiopia, 2019. Method: Institutional unmatched case-control study design was employed on 310 mothers who gave full-term births as control and 104 mothers with preterm births as cases from October 1, 2018, to February 1, 2019. The study participants were selected using a consecutive sampling method. SPSS version 20 was used to analyze the data. Result: A total of 104 (25.1%) cases and 310 (74.9%) controls were included in the analysis with a 100% response rate. Factors such as rural resident, AOR =2.7; 95% CI [1.3, 5.6], uneducated, AOR= 2.6; 95% CI [1.3, 5.2], ANC visits <4 times, AOR =5.5; 95% CI [2.1, 14.3], substance abuse, AOR =3.5; 95% CI [1.1, 10.5], MUAC <23cm, AOR= 7.2; 95%CI [3.3, 15.7], pregnancy-induced hypertension, AOR =8.9; 95%CI[1.2, 69], history of abortion, AOR =11.1; 95%CI[1.2, 105], FCS <=21.4, AOR =20.7; 95%CI[10, 42.2], and low birth weight, AOR= 20.2; 95%CI[10.5, 39] were identified as significant determinants of preterm birth. Conclusion: Education level, rural residence, Poor antenatal care visits, substance abuse, pregnancy-induced hypertension, and history of abortion are the key factors associated with pre-term birth. Community awareness and mobilization should be strengthened through extension programs. Emphasis should be given to strengthening efforts on the availability of basic health services and promoting education on nutrition during pregnancy, especially in rural areas. Further study is recommended regarding the effect of maternal nutrition on preterm birth using a prospective study design.