Adai (1390–1438) was the Mongol Khan of the Northern Yuan dynasty based in Mongolia. After the prominent eastern Mongolian chancellor, Arughtai, threw his allegiance to him, he briefly reunited most of the Mongols under his banner.

The origin of Adai’s family lineage traced back to Kadan and Hasar due to the interfamily marriages within the Borjigin clan. Mongolian sources also say Adai was a son of Örüg Temür Khan. Adai’s family was a member of eastern Mongol clans, Khorchin, originated in the region of Nen River to the east of Greater Khingan Range. Even before his proclamation as the Khagan, Adai almost succeeded in unifying western Mongol territory by defeating the Oirats. Alarmed by the possible resurgence of another Genghis Khan’s era, the Ming Dynasty provided support to the Oirats and their allies among the western Mongol clans, successfully turning the tide by first recovering and then launching counteroffensives against eastern and central Mongol clans.

Arughtai chingsang installed Adai on the throne as Khagan of the Great Yuan after discussing the matter with the family of Khasar.

During his rule, Adai was able to consolidate and expand his power, eventually unifying both the central and eastern Mongol territories in 1425. However, his conquest of western Mongol territory was checked by the Oirats and both sided continued the war for unification for decades. Riding on the success of unifying central and eastern Mongol territories, Adai proclaimed himself as Khagan with support of central and eastern Mongol clans in 1425, the same year his rivalry khan in the west, Oyiradai was killed. Adai and Arughtai crushed the Oirats and killed several leaders of them. After capturing the Oirat nobles, Adai, married Gulichi's wife, Öljeitü the Beauty, who had been a consort of Elbeg Khan (r.1392–1399) and enslaved Bahamu's son (future Toghan taishi). Although Adai Khan’s authority did not fully reach western Mongols who did not recognize him as supreme ruler, western Mongol clans did not have a khan of their own so Adai Khan remained as the sole Mongol ruler for the next eight years at least in name. Elbeg's daughter and the Oirat taishi Bahamu's widow, Samar, persuaded the Khan to release her son now named Toghan to western Mongolia. However, in 1433, Toghan raised a rebellion and western Mongols finally crowned Toghtoa Bukha (or Toγtoγa Buqa) with the title of Tayisung Khan as their next new khan, which resulted in half a decade of the simultaneous existence of two khans supported by opposing Mongol clans.

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