Based on fieldwork, archival, literary, and electronic sources, the article explores the phenomenon of interethnic mixing of Russians and Buryats in the southeast of Eastern Siberia — the settled indigenous people, the Karyms. Ideographic and comparative methods have allowed considering this topic from the imperial era to the 21st century. Since the late 17th century, the region became an arena for interaction between two peoples, races, language families, cultures, and religions. The nomadic experience of the Buryats provided cultural receptions for the adaptation of Russians. It is revealed that imperial policies regarding the Karyms, on one hand, supported internal isolation of traditional Buryat and Russian societies, while, on the other hand, implementing a gradual ac-culturation of some baptized Buryats who transitioned, thanks to interethnic marriage, to a settled way of life. Three stages in the formation of the Karyms identity are identified. The first, imperial stage, involved the formation of an interethnic group of indigenous people with special rights and duties, leading a settled lifestyle as peasant-farmers under Steppe councils. Isolation and the border position shaped an ethnic group with a common origin, culture, language, and self-awareness. The second stage was a period of voluntary assimilation: during Soviet times, the Karyms reasonably identified themselves as Russians. The third stage, in the 21st century, sees the Karyms descendants striving to officially assert themselves. The correlation between the concepts of “Karym” and “metis” is examined, proving their ambiguity. The term “metis” denotes a borderline anthropological type, referring to an individual’s phenotype. The Karyms represent a phenomenon of interethnic mixing between Russians and the Buryats in the southeast of Eastern Siberia, the result of one facet of Russia’s colonial policy that contributed to stabilizing the ethno-political and demographic situation.