Objective: To study the association between the risk of severe maternal morbidity and adequacy of prenatal care in teenage pregnancies. Design: Population-based retrospective cohort study. Setting: National Health Insurance Service National Delivery Cohort in Korea Population: A total of 23,202 delivery cases including pregnant teenagers aged 13 to 19 years and gestational age ≥ 37 weeks between 2003 and 2018. Methods: We used a generalized estimating equation model with adjustment for all covariates to determine the adjusted relative risk (RR) associated with severe maternal morbidity. Main Outcome Measures: severe maternal morbidity, Kessner Adequacy of Prenatal Care Index Results: Severe maternal morbidity occurred in 723 (3.1%) of the 23,202 delivery cases investigated, over a 16-year period. The risk of severe maternal morbidity was 1.8-fold higher in teenagers with inadequate prenatal care (RR 1.81, 95% confidence interval [CI] 1.39– 2.37) and 1.6-fold higher in those with intermediate prenatal care (RR 1.59, 95% CI 1.33–1.87) than in those with adequate prenatal care. Synergistic effects of inadequate prenatal care and maternal comorbidity were observed on severe maternal morbidity. Conclusion: This study confirmed that inadequate prenatal care was associated with the risk of severe maternal morbidity among pregnant teenagers. Notably, maternal comorbidity and inadequate prenatal care produced synergistic effects on severe maternal morbidity. Therefore, public health policy makers should focus on the development and implementation of programs to ensure adequate prenatal care and support financial accessibility of healthcare to improve teenage mothers’ health.