Mahmud Ghazan (also Ghazan Khan, sometimes westernized as Casanus; 5 November 1271 – 11 May 1304) was the seventh ruler of the Mongol Empire's Ilkhanate division in modern-day Iran from 1295 to 1304. He was the son of Arghun, grandson of Abaqa Khan and great-grandson of Hulegu Khan, continuing a long line of rulers who were direct descendants of Genghis Khan. Considered the most prominent of the Ilkhans, he is perhaps best known for converting to Islam and meeting Imam Ibn Taymiyya in 1295 when he took the throne, marking a turning point for the dominant religion of the Mongols in West Asia.

One of his many principal wives was Kököchin, a Mongol princess (originally betrothed to Ghazan's father Arghun before his death) sent by his great-uncle Kublai Khan.

Military conflicts during Ghazan's reign included war with the Mamluk Sultanate for control of Syria and battles with the Turko-Mongol Chagatai Khanate. Ghazan also pursued diplomatic contacts with Europe, continuing his predecessors' unsuccessful attempts at forming a Franco-Mongol alliance. A man of high culture, Ghazan spoke multiple languages, had many hobbies, and reformed many elements of the Ilkhanate, especially in the matter of standardizing currency and fiscal policy.

Ghazan's parents were Arghun and his concubine Kultak Egechi of the Dörböd. At the time of their marriage, Arghun was 12. Kultak's elder sister Ashlun was the wife of Tübshin, son of Hulagu and the previous viceroy in Greater Khorasan. According to Rashid al-Din Hamadani, the marriage took place in Mazandaran, where Arghun was viceroy.

Ghazan was born on 5 November 1271 in Abaskun (now near Bandar Torkaman), although he was raised in the nomadic palace of the orda of his grandfather Abaqa's favorite wife, Buluqhan Khatun, who herself was childless.^{[full citation needed]} Ghazan and Arghun didn't see each other until Abaqa's attack on Qara'unas in 1279, when they briefly met.

Ghazan was raised an Eastern Christian, as was his brother Öljaitü. The Mongols were traditionally tolerant of multiple religions, and during Ghazan's youth, he was educated by a Chinese Buddhist monk, who taught him Old Mandarin and Buddhism, as well as the Mongolian and Uighur scripts.

He lived together with Gaykhatu in Buluqhan Khatun's encampment in Baghdad after Abaqa's death. He reunited again with his father when Buluqhan Khatun was wed to Arghun and became Ghazan's step-mother.

After the overthrow of Tekuder in 1284, Ghazan's father Arghun was enthroned as Ilkhan, the 11-year-old Ghazan became viceroy, and he moved to the capital of Khorasan, never to see Arghun again. Emir Tegene was appointed as his deputy, who he didn't like very much. In 1289, conflict with other Mongols ensued when a revolt was led against Arghun by Nawruz, a young emir of the Oirat clan, whose father had been civil governor of Persia before the arrival of Hulegu. Ghazan's deputy Tegene was among the victims of Nawruz's raid on 20 April 1289 in which he was captured and imprisoned. Nawruz's protege, Prince Hulachu was arrested by Ghazan's commander Mulay ten days later. When Nawruz was defeated by Arghun's reinforcements in 1290, he fled the Ilkhanate and joined the alliance of Kaidu, another descendant of Genghis Khan who was the ruler of both the House of Ögedei and the neighboring Chagatai Khanate. Ghazan spent the next ten years defending the frontier of the Ilkhanate against incursions by the Chagatai Khanate of Central Asia.