Arunāchal Pradesh, state, located in northeastern India, bordered on the south by Assam state, on the west by Bhutan, on the north and northeast by China, and on the east by Myanmar (formerly known as Burma). Arunāchal Pradesh (Sanskrit for 'Land of the Rising Sun') has an area of 83,743sq km (32,333 sq mi). A portion of the state’s territory is claimed by China.

The terrain rises through a series of foothills in the south to the Lesser Himalayas and, on the Tibetan border, to the ranges of the Great Himalayas. The state's main rivers are the Brahmaputra, known in Arunāchal Pradesh as the Siang, and its tributaries, the Tirāp, the Lohit (Zayü Qu), the Subansirī, and the Bhareli. The climate of the foothills is subtropical; in the mountains, temperatures decrease rapidly with altitude. Rainfall averages between 2,000 and 4,000 mm (80 and 160 in) a year. The state's diverse terrain and climate is reflected in its plant and animal life. There are more than 500 species of orchids, and the abundant forests range from alpine to subtropical. Animal species include tiger, leopard, elephant, red panda, and deer.

In 2001 Arunāchal Pradesh had a population of 1,091,117 and an average density of 13 persons per sq km (34 per sq mi). Itānagar is the capital. There are 20 main ethnic groups and more than 50 languages in Arunāchal Pradesh, including Assamese, Hindi, and English, all of which are used as lingua francas, or common languages. Animism (the worship of nature deities and other spirits) is the dominant form of religion, but there are also strong Hindu and Buddhist traditions. The Buddhist monastery in Tawang, dating from the 17th century, is one of the largest in India. The monastery was the birthplace of the sixth Dalai Lama, the spiritual leader of Tibetan Buddhism.

The economy of Arunāchal Pradesh is predominantly agricultural and the principal crop is rice. Maize, millet, wheat, pulses (edible seeds gathered from pea and bean crops), potatoes, sugarcane, fruit, and oilseeds are also important. Many areas depend on shifting cultivation (also known as slash-and-burn agriculture), in which trees and grasses are burned from an area so a crop may be planted for several seasons and then shifted to a new area. Industry is small scale and includes timber, rice, and oil mills; soap and candle making; sericulture (raising silkworms for the production of raw silk); and handicrafts. The economic potential of Arunāchal Pradesh's forests and rivers, and of its coal, oil, and other mineral deposits has yet to be exploited, partly because rough terrain makes transportation difficult. In 1992 the state was opened up to limited tourism.

Arunāchal Pradesh has a single-chamber Legislative Assembly, which has 60 seats. The state sends three members to the Indian national parliament: one to the Rajya Sabha (upper house) and two to the Lok Sabha (lower house). Local government is based on 12 administrative districts.

The region that is now Arunāchal Pradesh is mentioned in the Puranas (Sanskrit writings about the beginning of time), but little else is known of the state's early history. Part of Arunāchal Pradesh was annexed by the Ahom kings of Assam in the 16th century. In 1826 Assam became part of British India, but efforts to bring Arunāchal Pradesh under British administration did not begin until the 1880s. In 1912 the region became an administrative unit within Assam, called the North Eastern Frontier Tract (NEFT); in 1954 the NEFT became the North East Frontier Agency. Its northern boundary with Tibet has been disputed since 1913, when China rejected British proposals that the border should follow the crest of the Himalayas. This proposed border, known as the McMahon line, has served as the de facto boundary since. Following Indian independence in 1947, China laid claim to almost the whole area of Arunāchal Pradesh. Between 1959 and 1962 Chinese troops crossed the McMahon line several times, temporarily seizing border posts and capturing Indian troops. In 1962 the Chinese withdrew from Arunāchal Pradesh. Despite many attempts to negotiate a settlement, this border dispute has not yet been resolved. The region became the union territory of Arunāchal Pradesh in 1972, and India's 24th state in December 1986.