Aim. To study the theory of the origin and development of ethnic groups, as well as their role in teachings, in the writings of the church father Augustine, who lived in the period of late antiquity and whose views determined the doctrine of the Catholic Church (Augustinianism) from the fifth century to the era of the “high” Middle Ages. Methodology and Approach. Using systemic and terminological analysis, Augustine’s chief work, De Civitate Dei, as well as his treatises Enhiridion and On the Consent of the Evangelists, are studied for their ideas about ethnic groups and their role in society. Results. Ethnoses (gentes) in the teaching of Augustine are blood-related tribal collectives distinguished from one another by their origin with different biblical ancestors, their language differences and their settlement territories. In his work, ethnic groups differ from peoples understood as a collection of citizens, subjects of the state or a community ordered by laws: peoples constitute the states of the earthly city, whereas ethnic groups are elements of the City of God, connected to it through the institution of the Christian church. Relations between nations are considered by Augustine from the standpoint of the spread of Christianity and expansion of the church. The military campaign of the Germans against the Roman Empire, as well as his own concept of history, predetermined Augustine’s attitude to wars as an inevitable evil and his development of the principles and practice of the just war in international and interethnic interactions. Theoretical and Practical implications. Issues of ethnic community in Augustine’s theory, as well as his ideas about the role of ethnic groups in statehood and interstate and international interaction, seem important for understanding the content of the early Christian doctrine he promulgated, as well as the European worldview that developed in the early Middle Ages. These issues have not been specifically studied in either Russian or foreign literature, despite a significant amount of extant research on the theoretical legacy of Augustine.