Katihar district is one of the thirty-eight districts of Bihar state, India, and Katihar town is the administrative headquarters of this district. The district is a part of Purnia Division.

Katihar became a district when it was split from Purnia in 1973.Earlier, Katihar district was dominated by the Choudhary family, who were the biggest landlords of Kosi zone. The founder of Choudhary family was Khan Bahadur Choudhary Mohammad Baksh and he had an authority over 126 small landlords of Kosi zone. He died in 1943. The family holds lands of about 15,000 acres in Katihar district, and 8,500 acres in Purnia.

Katihar district occupies an area of 3,057 square kilometres (1,180 sq mi), comparatively equivalent to Canada’s Akimiski Island.

Katihar district is situated in the plains of North Eastern part of Bihar State, surrounded by Purnea district (Bihar) in the north and the west, Bhagalpur district (Bihar) and Sahebganj district (Jharkhand) in the south and Malda district and Uttar Dinajpur district (Paschim Bengal) in the east.

In 2006 the Ministry of Panchayati Raj named Katihar one of the country’s 250 most backward districts (out of a total of 640).It is one of the 36 districts in Bihar currently receiving funds from the Backward Regions Grant Fund Programme (BRGF).

According to the 2011 census Katihar district has a population of 3,068,149, roughly equal to the nation of Oman or the US state of Iowa.This gives it a ranking of 117th in India (out of a total of 640).The district has a population density of 1,004 inhabitants per square kilometre (2,600/sq mi).Its population growth rate over the decade 2001–2011 was 28.23%.Katihar has a sex ratio of 916 females for every 1000 males, and a literacy rate of 53.56%.The district has a significant concentration of Muslims.

Surjapuri language, north Bengal’s native language is spoken in Barsoi subdivision of Katihar district while Bengali language is also significantly spoken in Katihar.