The region that is now Arunachal 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 Arunachal 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 Arunachal 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. After the independence of India in 1947, China made claims to practically the whole area covered by the districts of East and West Kameng, Lower and Upper Subansiri, East and West Siang, and Lohit, arguing that the McMahon Line had never been accepted by China and was the result of British "aggression." Following this dispute, Chinese troops crossed the McMahon Line on August 26, 1959, and captured an Indian outpost at Longju, a few miles south of the line. They abandoned this in 1961 but in October 1962 crossed the line, this time in force. After first striking toward the Tanglha ridge and Tawang near the Bhutan border, the Chinese later extended their attack along the whole frontier. Deep inroads were made at a number of points. Later the Chinese agreed to withdraw approximately to the McMahon Line and in 1963 returned Indian prisoners of war

The region became the union territory of Arunachal Pradesh in 1972, and India's 24th state in December 1986.