In 1999, during the excavation of Khaiyrgas Cave on the Middle Lena River in Yakutia (in the Russian Federation), a fragmented human deciduous tooth was discovered in the upper 5th (Paleolithic) horizon, at the contact with the 4th (Mesolithic) horizon. The cave is one of a number of interesting Upper Paleolithic sites on the Lena, located at the border of two cultural regions: Lake Baikal and Yakutia. Previous findings indicate active cultural and perhaps ethnic contacts, in particular with areas of the Lower Angara region and Northern Baikal. In order to determine the morphology and taxonomic position of the tooth, its metric and non-metric parameters were studied. Comparative analysis indicates that the tooth characteristics adhere to the Eastern dental pattern with some archaic traits. This tooth is one of the earliest known human remains in Yakutia, which are crucial for the study of the ancient population of the Upper Palaeolithic and Mesolithic in North-East Asia. Its conformity with the Eastern dental pattern confirms the alleged affiliation of human groups that inhabited the territory of Yakutia in the late Pleistocene/early Holocene to the Mongoloid anthropological type. In addition, the results of comparative analysis suggest the presence of ancient affinities between the Upper Paleolithic populations of North-Eastern and Western Siberia.