Alauddin Khalji (Persian: علاء الدین خلجی; born Ali Gurshasp; c. 1266 – 4 January 1316) was the Khalji dynasty Sultan of Delhi from 1296 until his death in 1316. Alauddin instituted a number of significant administrative changes in the Delhi Sultanate, related to revenues, price controls, and society. He also successfully fended off several Mongol invasions of India.

Alauddin was a nephew and a son-in-law of his predecessor Jalaluddin. When Jalaluddin became the Sultan of Delhi after deposing the Mamluks, Alauddin was given the position of Amir-i-Tuzuk (equivalent to master of ceremonies). After suppressing a revolt against Jalaluddin, Alauddin obtained the governorship of Kara in 1291, and the governorship of Awadh in 1296, after a profitable raid on Bhilsa. In 1296, Alauddin raided Devagiri, and used the acquired loot to stage a successful revolt against Jalaluddin. After killing Jalaluddin, he consolidated his power in Delhi, and subjugated Jalaluddin's sons in Multan.

Over the next few years, Alauddin successfully fended off the Mongol invasions from the Chagatai Khanate, at Jaran-Manjur (1297–1298), Sivistan (1298), Kili (1299), Delhi (1303), and Amroha (1305). In 1306, his forces achieved a decisive victory against the Mongols near the Ravi riverbank, and later ransacked the Mongol territories in present-day Afghanistan. The military commanders that successfully led his army against the Mongols include Zafar Khan, Ulugh Khan, and Alauddin's slave-general Malik Kafur.

Alauddin conquered the kingdoms of Gujarat (raided in 1299 and annexed in 1304), Jaisalmer (1299), Ranthambore (1301), Chittor (1303), Malwa (1305), Siwana (1308), and Jalore (1311). These victories ended several Rajput and other Hindu dynasties, including the Paramaras, the Vaghelas, the Chahamanas of Ranastambhapura and Jalore, the Rawal branch of the Guhilas, and possibly the Yajvapalas. His slave-general Malik Kafur led multiple campaigns to the south of the Vindhyas, obtaining a considerable amount of wealth from Devagiri (1308), Warangal (1310) and Dwarasamudra (1311). These victories forced the Yadava king Ramachandra, the Kakatiya king Prataparudra, and the Hoysala king Ballala III to become Alauddin's tributaries. Kafur also raided the Pandya kingdom (1311), obtaining much treasure, elephants, and horses.

During the last years of his life, Alauddin had an illness and relied on Malik Kafur to handle the administration. After his death in 1316, Malik Kafur appointed Shihabuddin, son of Alauddin and his Hindu wife Jhatyapali, as a puppet monarch. Alauddin's elder son Qutbuddin Mubarak Shah seized power shortly after his death.

Contemporary chroniclers did not write much about Alauddin's childhood. According to the 16th/17th-century chronicler Haji-ud-Dabir, Alauddin was 34 years old when he started his march to Ranthambore (1300–1301). Assuming this is correct, Alauddin's birth can be dated to 1266–1267. His original name was Ali Gurshasp. He was the eldest son of Shihabuddin Mas'ud, who was the elder brother of the Khalji dynasty's founder Sultan Jalaluddin. He had three brothers: Almas Beg (later Ulugh Khan), Qutlugh Tigin, and Muhammad.

Alauddin was brought up by Jalaluddin after Shihabuddin's death. Both Alauddin and his younger brother Almas Beg married Jalaluddin's daughters. After Jalaluddin became the Sultan of Delhi, Alauddin was appointed as Amir-i-Tuzuk (equivalent to Master of the Ceremonies), while Almas Beg was given the post of Akhur-Beg (equivalent to Master of the Horse).

Alauddin married Jalaluddin's daughter, Malika-i-Jahan, long before the Khalji revolution of 1290. The marriage, however, was not a happy one. Having suddenly become a princess after Jalaluddin's rise as a monarch, she was very arrogant and tried to dominate Alauddin. According to Haji-ud-Dabir, Alauddin married a second woman, named Mahru, who was the sister of Malik Sanjar alias Alp Khan. Malika-i-Jahan was greatly infuriated by her husband taking a second wife. According to Dabir, this was the main cause of the misunderstanding between Alauddin and his first wife. Once, while Alauddin and Mahru were together in a garden, Jalaluddin's daughter attacked Mahru out of jealousy. In response, Alauddin assaulted her. The incident was reported to Jalaluddin, but the Sultan did not take any action against Alauddin. Alauddin was not on good terms with his mother-in-law either, who wielded great influence over the Sultan. According to the 16th-century historian Firishta, she warned Jalaluddin that Alauddin was planning to set up an independent kingdom in a remote part of the country. She closely monitored Alauddin and encouraged her daughter's arrogant behavior toward him.