Abtai Sain Khan (Mongolian: ᠠᠪᠲᠠᠶ ᠢᠰᠠᠶᠢᠨ ᠬᠠᠨ Абтай сайн хан; 1554–1588) – alternately Abatai or Avtai (Mongolian: Автай хан, meaning who have the gift of witchcraft (Автай) and good (сайн) – was a Khalkha-Mongolian prince who was named by the 3rd Dalai Lama as first khan of the Tüsheet Khanate in 1587. He zealously propagated Tibetan Buddhism among the Khalkha Mongols and founded the Buddhist monastery of Erdene Zuu in 1585.
Abtai was born in 1554 to the Khalkha Mongol prince Onokhui üizen Noyan (b. 1534). He was the eldest among Onokhui üizen Noyan's five sons which include Abugho, Tarni, Tumengken, and Barai. He was the great-grandson of Batu Mongke Dayan Khan (1464–1517 / 1543) and grandson of Gersenji (1513–1549). Blood smeared on his fingers at birth presaged a great warrior and from 1567 to 1580 Abtai led several campaigns against western Oirat Mongol tribes, finally defeating the Oirats' Khoshut tribe at Köbkör Keriye in the mid-1580s. Abtai then placed his son Shubuudai on the Oirat throne. His ferociousness in battle earned him the moniker The Mad Hero or the Mad Taiji of North Khalkha.