The article is devoted to the analysis of the Yakut ethnonym *J̌emkon and *Yemkon, which can be identified in the personal onomastics (the name Zhemkon and surname Yemkonov) and derived names of various administrativeterritorial units (Yemkonskaya or Yamkonskaya volost’, Zhemkonskiy nasleg) recorded in the Russian documents. For the first time, the ethnonym becomes known in the documents of 17th century related to the population of the Lena River valley, which later became part of the Kangalasskiy ulus. Later the name was recorded in Vilyuy region. The modern Yakut spelling cöppön, cökpön, cökkön, cömkön reflects a late adaptation in the Yakut environment. In this connection the authors of the article draw their attention to the ethnonym known among the Buryats. It is widespread in different versions over vast territories among several ethnic-territorial groups of the Buryats. It is form Zemχed (letter ǰimked) of the Khori (Qori) tribe Khudai (Qudai). The various forms of the ethnonym and the corresponding eponyms as Yenχen, Zenχen, Ženχen, Žinχen are widely represented both among the Khori (Qori) tribe Galzut (Γalzuud) settled not only among the Khori (Qori) and Aga Buryats, and among the Verkholensk, Ol’khon, and Barguzin ethno-territorial groups of Western Buryats. The analysis shows that the Russian spelling of the Yakut ethnonym could reflect an adaptation of the original variant to be close to the pronunciation of *ǰimken (~ *dʹimken?), which in turn makes it possible to characterize its phonetic appearance as tending to Western Buryat dialects. Therefore, there are reasons to associate the penetration of this ethnonym into the Yakut environment with native speakers of a Western Buryat type language. The etymology of the analyzed name is difficult. Formal analysis of morphology leads to the opinion that the possible reconstructed semantics would be more likely to a personal name or toponym. It is also noted that it is possible to consider the spelling *J̌imken ~ *Yimken as correspond in its Mongolic form to the name known from the sources of the 11th–14th centuries and related to the Turkic tribe Yemäk ~ Yimäk. However, the authors leave this remark only as a hypothesis to be possible for further discussions.