Kokrajhar district is an administrative district in Bodoland Territorial Region of Assam. It is predominantly inhabited by the Boro tribe. The district has its headquarters located at Kokrajhar Town and occupies an area of 3,169.22 km² (1,223.64 sq mi). It has two civil sub-divisions namely Parbatjhora and Gossaigaon and five revenue circles namely Kokrajhar, Dotma, Bhaoraguri, Gossaigaon and Bagribarilll.

From the map of 1776 Rennell, Kokrajhar was spelled CONKERAJHAR.

From early 17th-century present-day Kokrajhar district was under the control of Kingdom of Bhutan, till the Duar Wars in 1865 when British removed the Bhutanese influence and later the areas were merged to undivided Goalpara district of the Indian Union in 1949.

The Druk Desi (Dzongkha: འབྲུག་སྡེ་སྲིད་) of Bhutan appointed Paro Penlop to look after the Duars, who in turn appointed local people as Subah or Laskar, below this was an officer called Kamta who was appointed directly by the Deb Raja of Bhutan.

Kokrajhar was a part the undivided Goalpara district. In 1957, under the administration of Bimala Prasad Chaliha as the Chief Minister of Assam, three sub-divisions were created one of which was Kokrajhar. This sub-division was made into a district on 1 July 1983.

On 29 September 1989 Bongaigaon district was created from parts of Kokrajhar and Goalpara.

Kokrajhar district occupies an area of 3,129 square kilometres (1,208 sq mi), comparatively equivalent to Indonesia's Waigeo Island. Kokrajhar district is located on the northern bank of the Brahmaputra river. It forms the gateway to the Seven Sister States. Kokrajhar shares its boundary with Bongaigaon (now known as Chirang), Dhubri, West Bengal, Barpeta and Bhutan. Part of the district is made up of Manas National Park.

In 2006 the Indian government named Kokrajhar one of the country's 250 most backward districts (out of a total of 640). It is one of the eleven districts in Assam currently receiving funds from the Backward Regions Grant Fund Programme (BRGF).