The Chagatai language (جغتای Jağatāy; Uzbek: چەغەتاي Chag'atoy; Mongolian: ᠲᠰᠠᠭᠠᠳᠠᠢ, Chagadai ; Uyghur: چاغاتاي Chāghātāy； Turkish: Çağatayca) is an extinct Turkic, Mongolic language which was once widely spoken in Central Asia, and remained the shared literary language there until the early twentieth century. Soviet scholarship termed the Chagatai language "Old Uzbek."Ali-Shir Nava'i was the greatest representative of Chagatai language literature. Chagatai was also spoken by the early Mughal rulers in the Indian subcontinent.

The word Chagatai relates to the Chagatai Khanate, a descendant empire of the Mongol Empire, which was left to Genghis Khan's second son, Chagatai Khan. Many of the Chagatai Turk,Mongols and Tatars who were the speakers of this language claimed descent from Chagatai Khan.

Chagatai belongs to the Uyghur branch of the Turkic language family. It is descended from the Old Uyghur that served as a lingua franca in Central Asia, with a strong infusion of Arabic and Persian words and turns of phrase. It was developed as a sophisticated written language using the Persian alphabet. It can be divided into three periods: