Tien Shan, major mountain system, Central Asia, extending from the Pamirs northeast along the border between Kyrgyzstan, southeastern Kazakhstan, and the Xinjiang Uygur Autonomous Region of China. It is also known as Tian Shan. The Tien Shan (Chinese, «Heavenly Mountains») has a length of about 2414 km (about 1500 mi) and a width of about 320 to 480 km (about 200 to 300 mi); it covers an area (1,036,000 sq km/400,000 sq mi) approximately equal to that of the Rocky Mountains in the United States. In China it divides the Junggar Pendi (Dzungarian Basin) to the north from the vast, arid Tarim Pendi (Tarim Basin) to the south. The major rivers, including the Syr Darya, Ili (Yili), and Chu, flow generally westward. In the border area where Kazakhstan, Kyrgyzstan, and China meet is a string of high peaks, notably Victory Peak (7439 m/24,406 ft), the highest in the system, and Hantengri Peak (6398 m/20,991 ft), from which the 34-km (21-mi) long Muzart glacier descends. West of these peaks, at an altitude of 1607 m (5273 ft) is the Kyrgyz Lake Ysyk-Kцl (6100 sq km/2360 sq mi). In the eastern part of the range, the most striking feature is the Turpan Pendi (Turfan Depression), a 161-km (100-mi) long stretch of lowland reaching 154 m (505 ft) below sea level and enclosed by high mountains.

The ranges of the Tien Shan, which generally lie along an east-west axis, were lifted up by geologic folding during the Paleozoic era. The crystalline and sedimentary rock has been subject to extensive erosion and to deep faulting; severe earthquakes have occurred along the rim of the system throughout modern times. The ranges are steeply sloped, their crests often incised by glaciers that wind down toward intervening valleys. The largest glaciers occur at high altitudes along the international boundary, although glaciers 19 km (12 mi) in length are not unusual in the high eastern Tien Shan.

The northern slopes of the Tien Shan receive enough moisture to support deep evergreen forests and highland meadows suitable for grazing livestock. There, at an elevation of 853 m (2800 ft), the fertile Ili Valley lies within two arms of the system. The southern slopes of the mountain system are relatively dry and barren.

The several million inhabitants of the Chinese Tien Shan are largely Muslim, non-Chinese people, farmers and herders who speak the Uygur or Kyrgyz language; colonization by ethnic Chinese, however, is on the increase. On the western side of the border, in Kyrgystan and Kazakhstan the population is denser and industrialization more advanced; oil, coal, iron, and copper deposits are exploited. Livestock raising is the dominant agricultural occupation.