Yasa'ur (died 1320) was aChagatai prince who launched a revolt against theIlkhanAbu Sa'id. He was the son of Chübei, grandson ofAlghu Khan, and a great-great-grandson ofChagatai Khan.
Yasa'ur had originally resided within theChagataiulus. In 1314 he participated in a campaign against the Ilkhanate, together with the Chagatai Khan's brotherKebek and aNeguderi princeDaud Khwaja (a son ofQutlugh Khwaja) who had been expelled from his territories by the Ilkhan the previous year. Their forces defeated the Ilkhan army near the banks of theMurgab River; they then marched toHerat. The IlkhanÖljeitü then set off with an army for the east, while the Chagataiid forces were recalled by the khanEsen Buqa.
Around this time Yasa'ur, who had been accused by Kebek of cooperated with the Ilkhanid forces during the invasion, defected to Öljeitü. His forces engaged in battle with the Chagataiid troops; an Ilkhanid army that had crossed theOxus joined the fighting and secured victory for Yasa'ur. He, escorted by the Ilkhanid force, enteredKhorasan; Öljeitü then allowed him to take control of the pasture lands of theBadghis.
Öljeitü died in 1316 and was succeeded as Ilkhan by his son Abu Sa'id. Yasa'ur initially pledged loyalty to the new Ilkhan, but was apparently uncooperative with Ilkhanid authorities. In early 1319 he openly revolted against Abu Sa'id, invading the province ofMazandaran. This revolt coincided with an invasion byUzbeg Khan, forcing the Ilkhanids to defend against two attacks simultaneously. Amir Husain (the father ofHasan Buzurg, who founded theJalayirid dynasty inBaghdad) was sent to deal with Yasa'ur. The AmirCoban, overestimating the strength of Yasa'ur, also planned to march against the Chagataiid prince at the head of a 20,000-strong army, but eventually decided to defend against Uzbeg's forces.
Amir Husain's approach caused Yasa'ur to retreat from Mazandaran. FromTus he then sent a force to takeHerat, whose ruler, theKartid Giyath al-Din, had declared his loyalty to the Ilkhanids. The siege of Herat was unsuccessful, however, as upon hearing of the approach of the Ilkhanid army Yasa'ur withdrew into southernAfghanistan. In the following year, he was killed by Kebek, who had recently become Chagatai Khan and who deemed Yasa'ur to still be a threat.
Yasa'ur's personal troops eventually formed the basis of theYasa'uri, a group that gained influence within the Chagataiulus in the mid-fourteenth century.