The Juyan Lake (Chinese: 居延海; Wade–Giles: Chüyen Hai; Mongolian: Gashuun Nuur (shown on Chinese maps as 嘎顺淖尔 Gāshùn nào'ěr or 嘎顺诺尔, Gāshùn nuò'ěr) for western lake, Sogo Nuur for eastern lake) is a former lake in the Gobi Desert. It is located in the western part of Inner Mongolia, in Ejin Banner of the Alxa League, near the border with Mongolia. Gashuun Nuur had an area of 267 km (103 sq mi) in 1958, of 213 km (82 sq mi) in 1960, and dried up in 1961. The eastern lake reappeared in 2005. As of August 2012, the area of the lake is 38 km (15 sq mi).

The Juyan Lake basin is a rare wetland located in a desert. It covers an area of about 10,000 square kilometres (3,900 sq mi). The Juyan lake is one of three former terminal lakes located at the outer edges of the Heihe River ('Black River') catchment which formed a large inland delta between the Qilian and the Gobi Altay. The Heihe River is also known as the Ruo Shui (Chinese: 弱水; lit. 'weak river'), also Etsin Gol or Ruo He or Ejin River.

The basin's boundary is formed by the Mazong Shan mountains to the west, the Heli Shan and Longshou Shan mountains to the south, the Helan Mountains and Lang Shan ranges to the east and the Gobi Altay range to the north.

The basin played an important role in ancient times and was historically part of the Hexi Corridor between the 2nd century BC to the 8th century AD.

"This territory, called Juyan by the Han Chinese, was maintained and garrisoned by the empire from the time of Emperor Wu until the last century of Later Han. Militarily, the outpost of the Great Wall was important for two reasons: as a supply point for the garrisons in the northwest and, perhaps more significant, as a means to deny this prosperous region to the northern nomads. Left undefended, Juyan would have provided an ideal route for attack against the Chinese commanderies of the corridor itself.

During Former Han, therefore, the Zhelu Zhang (Fortress to Block the Enemy), had been constructed by the marshes of the Edsin Gol, and it was from this base, for example, that the general Li Ling went forth on his disastrous attack against the Xiongnu in 99 BC.

South of the Juyan salient, the main line of defences followed the Great Wall, which ran in this region from the passes of Yumen Guan and Yang Guan in Dunhuang commandery of the far west along the northern edge of the Hexi Corridor past Jiuquan, Zhangye and Wuwei."

In 1930, the Sino-Swedish Expedition excavated ten sites in the Juyan Lake Basin and unearthed a total of 10,200 wooden slips dating to the Western Han, which came to be known as the "old Juyan texts." In 1937, after the Second Sino-Japanese War began, Chung-Chang Shen transported these wooden slips from Beijing to the University of Hong Kong. Another 20,237 slips were excavated between 1972 and 1976 by the Juyan Archaeological Team, Gansu. These slips are held by the Provincial Museum of Gansu and came to be known as the "new Juyan texts."

The science fiction series Perry Rhodan features the rocket-shuttle Stardust returning from the Moon in 1971 with alien technology, landing near the point where the Ejin River (called Edsengol) flows into the Juyan Lake (called Goshunsee, i.e. Goshun Lake), not far from the real-life Jiuquan Satellite Launch Center. The area, in which Earth's later capital Terrania is built, remains a central location in the series.

• Rafe de Crespigny. 1984. Northern Frontier. The Policies and Strategies of the Later Han Empire. Faculty of Asian Studies, Australian National University. Canberra.
• Wünnemann, B., Hartmann, K. (2002), Morphodynamics and Paleohydrography of the Gaxun Nur Basin, Inner Mongolia, China in: Zeitschrift für Geomorphologie, vol. 126, pp. 147–168.

Wikimedia Commons has media related to Juyan Lake.

• Paleoenvironment in the lower reaches of Hei River and Juyan Lake
• Lake Level Changes Recorded by Tree Rings of Lakeshore Shrubs
• Juyan Site
• Perrypedia page about the lake in the Perry Rhodan series