East Turkestan or East Turkistan (Uyghur: شەرقىي تۈركىستان, ULY: Sherqiy Türkistan, UKY: Шәрқий Туркистан) is a loosely defined geographical region in the northwestern part of the People's Republic of China, on the cross roads of East and Central Asia. The term was coined in the 19th century by Russian Turkologists, including Nikita Bichurin, who intended the name to replace the common Western term for the region, "Chinese Turkestan", which referred to the Tarim Basin in Southern Xinjiang. Beginning in the 17th century, Altishahr, which means "Six Cities" in Uyghur, became the Uyghur name for the Tarim Basin. Uyghurs also called the Tarim Basin "Yettishar," which means "Seven Cities," and even "Sekkizshahr", which means "Eight Cities" in Uyghur. Chinese dynasties from the Han dynasty to the Tang dynasty had called an overlapping area the "Western Regions".

Starting in the 20th century, the Turkic Muslim Uyghur separatists and their supporters used East Turkestan as an appellation for the whole of Xinjiang (the Tarim Basin and Dzungaria) or for a future independent state in present-day Xinjiang Uyghur Autonomous Region. They reject the name Xinjiang (meaning "New Frontier" in Chinese) because of the Chinese perspective reflected in the name, and prefer East Turkestan to emphasize the connection to other, western Turkic groups.

The First East Turkestan Republic existed from November 12, 1933, to April 16, 1934, and the Second East Turkestan Republic existed between November 12, 1944, and June 27, 1946. East Turkestan is a founding member of the Unrepresented Nations and Peoples Organization (UNPO) formed in 1991, where it was represented initially by the East Turkistan National Congress and later by the World Uyghur Congress post 2004. In September 2004, the East Turkistan Government in Exile was established in Washington, D.C.

The term "East Turkestan" was coined in the 19th century by Russian Turkologists, including Nikita Bichurin, who intended for the name to replace the common Western term for the region, "Chinese Turkestan", which referred to the Tarim Basin in southern Xinjiang or Xinjiang as a whole. The term "Uyghuristan" means 'land of Uyghurs'. The latter name was given to the region by medieval Muslim geographers.

In China, the term Western Regions (Chinese: 西域; pinyin: Xīyù; Wade–Giles: Hsi¹-yü⁴; Uyghur: Qurighar, Қуриғар) referred to the regions west of the Yumen Pass. During the 2nd century BC, Chinese writers called the city states of the northern and southern rim of the Taklamakan Desert the "Walled city-states of the Western Regions". In 60 BC, the Han dynasty established the Protectorate of the Western Regions "in charge of all the oasis-states of the Tarim Basin" which they ruled in indirectly with small agricultural military colonies. The Protector General was stationed at Wulei (west of Karasahr). The city-states were able to conduct their own independent policies with each other. After the Western Han period that ended in 9 AD, China lost its authority over the Western Regions until it was restored in 94 AD and officially lasted until 107 AD. After the crackdown of internal separatist forces, the Eastern Han dynasty set up another protectorate known as the Chief Official of the Western Regions instead. Since the Han, successive Chinese governments had to deal with secessionist movements and local rebellions from different peoples in the region. However, even when Xinjiang was not under Chinese political control, Xinjiang has long had "close contacts with China" that distinguish it from the independent Turkic countries of Central Asia. A Sogdian sale contract of a female slave from the period of the Gaochang kingdom under the rule of Qu clan from 639 CE mentioned the Sogdian word "twrkstn", which may have referred to the lands to the east and north of Syr Darya in the realm of the First Turkic Khaganate.

The Gökturks, known in ancient Chinese with pronunciation as Tutkyud as well as modern Chinese pronunciation as Tujue (Tu-chueh; Chinese: 突厥; pinyin: Tūjué; Wade–Giles: T'u¹-chüeh²), united the Turkic peoples and created a large empire, which broke into various Khanates or Khaganates; the Western Turkic Khaganate inherited Xinjiang, but West Tujue became part of China's Tang dynasty until the ninth century. However, the terms for West Tujue and East Tujue do not have any relation with the terms West and East Turkestan. "Turkestan", which means "region of the Turks", was defined by Arab geographers in the ninth and tenth centuries as the areas northeast of the Sir River. For those Arab writers, the Turks were Turkic-speaking nomads and not the sedentary Persian-speaking oasis dwellers. With the various migrations and political upheavals following the collapse of the Gökturk confederation and the Mongol invasions, "Turkestan", according to the official Chinese position, gradually ceased to be a useful geographic descriptor and was not used.

During the sixteenth century, the Chagatai Khanate completed the Islamification and Turkification of western Xinjiang and the surrounding region, known then as Moghulistan, while China's Ming dynasty held the Eastern Areas. After the Fall of the Ming dynasty, a western Mongol group established a polity in "Chinese Tartary", as it was sometimes known, or in eastern Xinjiang, expanding southward into southern Xinjiang. In 1755, the Qing dynasty defeated the Mongol Dzungar Khanate and captured two territories in Xinjiang. The northern territory, where the Dzungars lived, was called Dzungaria, while the southern areas, which the Dzungars controlled and mined, were called Huijiang (Hui-chiang; Chinese: 回疆; pinyin: Huíjiāng; Wade–Giles: Hui²-chiang¹; lit. 'Muslim territory') or Altishahr. The term "Xinjiang", which, up until that time, simply meant all territories new to the Qing, gradually shifted in meaning for the Qing court to exclusively mean Dzungaria and Altishahr taken together. In 1764, the Qianlong Emperor made this use of Xinjiang as a proper name official and issued an imperial order defining Xinjiang as a "provincial administrative area". After General Tso (Tso Ts'ung T'ang) suppressed the Dungan revolt in 1882, Xinjiang was officially reorganized into a province and the name Xinjiang was popularized, superseding "Xiyu" in writing.

At the same time as the Chinese consolidation of control in Xinjiang, explorers from the British and Russian Empires explored, mapped, and delineated Central Asia in a competition of colonial expansion. Several influential Russians would propose new terms for the territories, as in 1805 when the Russian explorer Timovski revived the use of "Turkestan" to refer to Middle Asia and "East Turkestan" to refer to the Tarim Basin east of Middle Asia in southern Xinjiang or, in 1829, when the Russian sinologist Nikita Bichurin proposed the use of "East Turkestan" to replace "Chinese Turkestan" for the Chinese territory east of Bukhara. The Russian Empire mused expansion into Xinjiang, which it informally called "Little Bukhara". Between 1851 and 1881, Russia occupied the Ili valley in Xinjiang and continued to negotiate with the Qing for trading and settlement rights for Russians. Regardless of the new Russian appellations, the original inhabitants of Central Asia generally continued not to use the word "Turkestan" to refer to their own territories.