This paper presents some new linguistic data on Tuhan, a moribund Sayan Turkic language presently spoken by a handful of old speakers and semi-speakers in the county of Tsagaan Üür in northern Mongolia’s East Khövsgöl region. Linguistically, besides displaying typical Sayan Turkic characteristics, Tuhan preserves archaic features otherwise not documented in the rest of Sayan Turkic. At the same time, it shares some isoglosses with Tofan and others with Altay and Yenissey Turkic. Despite the very small number of its speakers, Tuhan appears to be a highly important language for comparative Turcology: a precious linguistic gem among South Siberian Turkic languages.