Zasagt Khan (Mongolian: засагт хаан; Chinese: 扎薩克圖汗), born Tümen (Mongolian: Түмэн; Chinese: 圖們), (1539–1592) was a khagan of the Northern Yuan dynasty, reigning from 1558^[1] to 1592. He was the successor of Darayisung Gödeng Khan and had direct rule over the Chahar. It was during his rule that the Mongols conquered the Daur and Evenk tribes.

Tümen, who was born in 1539, was the first of three sons of Darayisung Gödeng Khan. By being recognized as Khagan, Altan validated his authority.^[2] However, coordinating Altan Khan's actions, Tümen won over Uriyangkhai and Daur Mongols.^[3] His relatives, Abtai Khan and Khutughtai Sechen Khung Taiji, brought a large portion of the Four Oirats back into the Mongol fold. In addition to his successful invasions of the Ming dynasty, Tümen conquered Koko Nur and appointed his son ruler there.

Tümen and other Mongol princes decided to adopt Tibetan Buddhism. In 1576, Tümen was converted by Ilduni Sanggiduktshi Garma Lama into Buddhism. He assembled the Six Tumens, and codified laws.^[4] He made reforms on state laws and exempted the Mongol nobles from some taxes. He compiled a new code that was supposed to be based on Yekhe Zasag of Genghis Khan. Thenceforwards he was called Jasagtu, who made peace with the Right Wing Tumens and gave their leaders official titles. He compelled the three Jurchen tribes such as Jurjis in Manchuria and Yekhe Tungusians, to pay tribute.^[5]

Zasaghtu Khan died in 1592.

• List of khans of the Northern Yuan dynasty