The paper deals with migrations of ancient peoples across the territory of modern Buryatia and the Baikal region from the V century BC to about the II century AD. Based on archaeological and anthropological materials, as well as written sources, the formation of the ethnogenetic composition of the ancient population of the region is analyzed. It has been established that in the Neolithic era there was a movement towards Lake Baikal of groups of mixed Caucasoid-Mongoloid population from the West and Mongoloid populations from the East, from Transbaikalia and Mongolia. In the Bronze Age and Early Iron Age, a powerful migration flow of the Caucasoid population from Western Mongolia and Altai was recorded; they carried the culture of the Khereksurs barrows, to the territory of Northern Mongolia and Transbaikalia, where the Mongoloid population of the slab-grave culture lived. At the subsequent historical stage, the Xiongnu and Xianbei tribes actively interact in the region. The Xiongnu had a mixed Mongoloid-Caucasoid anthropological composition, with a significant predominance of Mongoloids. The Xianbrei were typical representatives of the Siberian Mongoloids. As a result, intensive metizational processes took place on the territory of Baikal Siberia, which led to the formation of medieval Turkic and Mongolian tribes, and later, modern peoples of the region.