Ngari Prefecture (Tibetan: མངའ་རིས་ས་ཁུལ།, Wylie: mnga' ris sa khul, ZWPY: ngari sakü) or Ali Prefecture (simplified Chinese: 阿里地区; traditional Chinese: 阿里地區; pinyin: Ālǐ Dìqū) is a prefecture of China's Tibet Autonomous Region covering Western Tibet, whose traditional name is Ngari Khorsum. Its administrative centre and largest settlement is the town of Shiquanhe. It is one of the least densely populated areas in the world, with 0.4 people per square kilometer (1.0 per square mile).

In ancient times, Ngari was known as Zhangzhung. The Zhangzhung kingdom extended over much of western Tibet, until its conquest by the Tibetan Empire under Songtsen Gampo. Zhangzhung sites, such as its capital Khyung-lung dngul-mkhar, are traditionally believed to be closely associated with the development of Bon, the indigenous Tibetan religion. According to Bon tradition, the religion first spread to Zhangzhung from the semi-mythical lands of Olmo Lungring and Tagzig. Thereafter, Bon doctrines were transmitted to central Tibet. Archaeological evidence demonstrated a highly distinctive cultural tradition in the prehistoric era.

In the mid-7th century, the Tibetan Empire annexed Zhangzhung and expanded its system of administrative divisions known as Tongde to western Tibet. Tibetan militias were deployed in newly established military garrisons, as Classical Tibetan became widely used across the region.

The name "Ngari" (mnga' ris), meaning "domain", arose during the Tibetan Era of Fragmentation. The Tibetan Empire fell to a civil war between rival heirs of the Yarlung dynasty in the 840s. In 925, Kyide Nyimagon, a descendant of the last emperor, founded the kingdom of Ngari Khorsum ("three divisions of Ngari"). At the time, his kingdom encompassed both present-day Ngari and Ladakh. After Kyide Nyimagon's death, the kingdom was divided into three, namely Purang-Guge, Maryul (later known as Ladakh) and Zanskar. Guge was the early center of the Second Diffusion of Buddhism into Tibet. In 988, the Guge king Songne abdicated to become a monk and took the name Yeshe-Ö. He founded the Tholing Monastery, which became the center of Tibetan Buddhist translation. At Tholing, the translator Rinchen Zangpo started the New Tantra Tradition School and initiated a major period of monastery building. In 1072, Purang and Guge were further split as two separate kingdoms. Purang was closely tied to the kingdom of Yatse, which was founded in the Himalayan regions to the south. The Kagyu sect became active in Guge in the late 12th century and Mount Kailash came to be a major pilgrimage destination, especially among followers of Milarepa.

Under Mongol rule, the Mangyul Gungthang kingdom rose to dominance from Lower Ngari. Backed by the Sakyapa, it defeated Purang/Yatse in 1252–1253, received direct recognition from the Mongols and held much power over Ngari. Gungtang suffered as the Yuan dynasty fell, while Purang changed hands multiple times towards the end of the 14th century. Around the same time, the Purang/Yatse polity collapsed and came under Guge rule.

Ngari entered a long period of decline starting in the late 14th century, possibly due to reduced precipitation over the region. No new monastery was constructed in Ngari's core region after 1500. The Gelug spread from central Tibet into Ngari from the 15th century onward, and gained the support of the Guge kings. Sectarian conflicts intensified between the Gelug and the Kagyu, which enjoyed the support of Ladakh. During this period, Ladakh steadily expanded and eventually managed to completely conquer Guge in 1630.

In the 1680s, as Ladakh allied with Bhutan, which adhered to the Drukpa Kagyu sect, in the latter's disputes with Tibet, the Ganden Podrang government of Tibet launched an expedition into Ngari. Led by general Ganden Tsewang, the force of Tibetan and Mongol cavalry was victorious at Purang, Rutog and the Ladakhi capital Leh, thus bringing Ngari under the control of central Tibet.

The Ganden Podrang appointed governors known as Garpöns to administer Ngari, who headquartered in Gartok. The region was divided into four Dzongs (Purang, Tsaparang, Daba and Rutog) and six Pöns. The former were headed by appointed officials while the latter were granted to local noble families. The name "Ngari Khorsum" was reinterpreted to mean Purang, Guge and Rutog, reflecting a much reduced extent of Ngari. In 1841, the Dogras invaded Ngari after having successfully conquered Baltistan and Ladakh. After more than a year of fighting in both Ngari and Ladakh, the war ended with the Treaty of Chushul, which restored the status quo.