Chagatai Khan (Mongolian: Цагадай, Tsagadai; c. 1183–1241 or 1242) was the second son of Genghis Khan and first khan and origin of the names of the Chagatai Khanate, Chagatai language and Chagatai Turks.
He inherited most of what are now the five Central Asian states after the death of his father and ruled until his death in 1241. He was also appointed by Genghis Khan to oversee the execution of the Yassa, the written code of law created by Genghis Khan, though that lasted only until Genghis Khan was crowned Khan of the Mongol Empire. The Empire later came to be known as the Chagatai Khanate, a descendant empire of the Mongol Empire.
The true founder of the state was Chagatai's grandson Alghu. The state was much less influenced by Islam than the Ilkhanate to the southwest, but there were Muslims within the state and some did convert. However, they kept to old nomadic traits much longer. Some historians have said this was a major reason for the decline in urbanism and agriculture in this area which is known to have occurred. The first ruler who actually converted to Islam was Mubarak Shah (note the Arab name). His conversion occurred in 1256. However, this was very problematic because in less than 30 years other rulers would renounce Islam and return to older beliefs, although Tarmashirin converted to Islam and tried to turn the dynasty back toward Islam. His conversion provoked a huge backlash from nomadic groups in the eastern part of the realm who eventually killed him in 1334. After his death the Chagatai state lost its status and disintegrated. Tamerlane would later marry into this family. By the early 16th century they had reasserted themselves in the region that is present-day Uzbekistan and maintained a realm there until the 18th century as the Shaybanid ruling house of the Uzbeks.