Changes in the territorial extension of the Mordovians over the millennial period (the 3^rd to the 13^th centuries) are discussed, and causes of the displacement are determined in the article. The three major periods are identified in the process: the 3^rd to the 7^th, the 8^th to the 11^th, and the 11^th to the 13^th centuries. The 3^rd century marked the appearance of ancient burial grounds containing decorations that are ethnically typical of the Mordovians. The heyday of the ancient Mordovian culture in the late 3^rd and first half of the 5^th centuries was accompanied by the expansion of its territory to the north. By the late 7^th – first half of the 8^th century, two options of the ancient Mordovian culture were formed: the Erzya in the north and the Moksha in the south. In the 8^th–11thcenturies, the Moksha Mordvins settlement territory shifted to the north-west, which could be associated with the danger from the Khazar Khanate. The territory of the Erzya Mordvins settlement did not change in the period. In the 11^th–13^th centuries, and the Mordovian population fell into the spheres of influence of Kievan Rus and Volga Bulgaria. The struggle between these states, and later the princely strife in Russia, led to the resettlement of the Mordovians to safer areas: the Moksha moved to the Moksha-Vad river interfluve, while the Erzya relocated to the Tyosha river basin.