Mu'in ad-Din Unur (Arabic: معين الدين أنر, romanized: Muʿīn ad-Dīn ʿUnur; died August 28, 1149) was the ruler of Damascus from 1140 to 1149.
When Zengi, the atabeg of Aleppo, besieged Damascus in 1135, Mu'in ad-Din was at the head of the army defending the city.
A more thorough alliance, to protect Damascus against Zengi, was negotiated during a visit by Mu'in ad-Din to Jerusalem, accompanied by the future chronicler Usamah ibn Munqidh.
Having established peace with Aleppo, Mu'in ad-Din set out to besiege Salkhad and Bosra, after their governor, Altuntash, allied with Jerusalem against him.
Mu'in ad-Din prepared for the inevitable siege, although he had hoped his former alliance with Jerusalem could be restored and that the crusade would attack some other city.
When the crusaders arrived in July, according to Ibn al-Qalanisi, Mu'in ad-Din "distinguished himself in combat with them, and displayed a valour, steadfastness and gallantry such as was never seen in any other, never wearying in repelling them nor taking respite from the struggle against them."
After returning to Damascus in July 1149, Mu'in ad-Din "ate a hearty meal, as was his usual custom, and was seized thereafter by a loosening of the bowels...From this resulted in the disease known as dysentery..." The emir died on August 28 and was buried in the university he had established in the city.
In addition to Ibn al-Qalanisi's favourable depiction of him, the Christian chronicler William of Tyre also speaks very highly of Mu'in ad-Din: he showed "sincere fidelity" in his negotiations with the Kingdom of Jerusalem, and was "a man of much wisdom and a lover of our people."