Longju or Longzu (Tibetan: གླང་བཅུ, Wylie: glang-bcu; Chinese: 朗久; pinyin: Lǎngjiǔ) is a disputed area in the eastern sector of the China–India border, controlled by China but claimed by India. The village of Longju is located in the Tsari Chu Valley 2.5 kilometres (1.6 mi) south of the town of Migyitun, considered the historical border of Tibet. The area of Longju southwards is populated by the Tagin tribe of Arunachal Pradesh.

India had set up a border post manned by Assam Rifles at Longju in 1959 when it was attacked by Chinese border troops and forced to withdraw. After discussion, the two sides agreed to leave the post unoccupied. India established a new post at Maja, three miles to the south of Longju, but continued to patrol up to Longju. After the 1962 Sino-Indian War, the Chinese reoccupied Longju and brushed off Indian protests.

Since the late 1990s and early 2000s, China has expanded further south, establishing a battalion post at erstwhile Maja. In 2020, China built a 100-house civilian village close to this location in disputed territory.

Longju is 2.5 kilometres (1.6 mi) south of the Tibetan frontier town of Migyitun (Tsari Town), along the Tsari Chu river valley. The area was historically populated by the Mara clan of the Tagin tribe of Arunachal Pradesh. The border between Tibet and tribal territory was at the Mandala Plain just outside the town of Migyitun.

There was a crossing on the river from its left bank to the right bank near Longju, which was needed to enter the tribal territory from the Tibetan side. When Bailey and Morshead visited the area in 1905, they found the bridge broken. The Tibetans were unable to repair it because it was built using tribal materials and techniques. Evidently, the Tibetan authority stopped at Migyitun.

On 28 August 1959, the Indian Prime Minister Nehru explained to the parliament that Longju was a five days march from Limeking which in turn was a 12 days march from the nearest road at Daporijo, a total of about three weeks. At the time the route passed through dense forests and consisted of indigenously built "ladder climbs" and bridges.

Administratively, for China, Longju is located in Shannan, Tibet, while for India, it is located in the Upper Subansiri district (previously called the Subansiri Frontier Division).

During the negotiations for the McMahon Line in 1914, the British Indian negotiators were cognizant of the fact that Migyitun was Tibetan and also that the neighbouring Dakpa Sheri mountain (to the west) was regarded by them as a holy mountain. Taking these factors into account, they promised that the border would be drawn short of the high ridge line and avoid including the annual pilgrimage route in Indian territory as far as practicable.