Background Addressing the problem of maternal mortality in Nigeria requires proper identification of maternal deaths and their underlying causes in order to focus evidence-based interventions to decrease mortality and avert morbidity. Objectives The objective of the study was to classify maternal deaths that occurred at a Nigerian teaching hospital using the WHO International Classification of Diseases Maternal mortality (ICD-MM) tool. Methods This was a retrospective observational study of all maternal deaths that occurred in a tertiary Nigerian hospital from 1.sup.st January 2014 to 31.sup.st December,2018. The WHO ICD-MM classification system for maternal deaths was used to classify the type, group, and specific underlying cause of identified maternal deaths. Descriptive analysis was performed using Statistical Package for Social Sciences (SPSS). Categorical and continuous variables were summarized respectively as proportions and means (standard deviations). Results The institutional maternal mortality ratio was 831/100,000 live births. Maternal deaths occurred mainly amongst women aged 25-34 years;30(57.7%), without formal education; 22(42.3%), married;47(90.4%), unbooked;24(46.2%) and have delivered at least twice;34(65.4%). The leading causes of maternal death were hypertensive disorders in pregnancy, childbirth, and the puerperium (36.5%), obstetric haemorrhage (30.8%), and pregnancy related infections (17.3%). Application of the WHO ICD-MM resulted in reclassification of underlying cause for 3.8% of maternal deaths. Postpartum renal failure (25.0%), postpartum coagulation defects (17.3%) and puerperal sepsis (15.4%) were the leading final causes of death. Among maternal deaths, type 1, 2, and 3 delays were seen in 30(66.7%), 22(48.9%), and 6(13.3%), respectively. Conclusion Our institutional maternal mortality ratio remains high. Hypertensive disorders during pregnancy, childbirth, and the puerperium and obstetric haemorrhage are the leading causes of maternal deaths. Implementation of evidence-based interventions both at the hospital and community levels may help in tackling the identified underlying causes of maternal mortality in Nigeria.