Samastipur is one of the thirty-eight districts of Bihar in India. The district headquarters are located at Samastipur. The district occupies an area of 2904 km² and has a population of 27,16,929 (as of 2001 Census).

Samastipur district occupies an area of 2,904 square kilometres (1,121 sq mi), comparatively equivalent to Indonesia’s Muna Island.Samastipur is bounded on the north by the Bagmati River which separates it from Darbhanga district. On the west it is bordered by Vaishali and some part of Muzaffarpur district, on the south by the Ganges, while on its east it has Begusarai and some part of Khagaria district. The district headquarters is located at Samastipur.

Samastipur is traversed by a number of rivers including Budhi Gandak, Baya, Kosi, Kamla, Kareh and Jhamwari and Balan, which are both the offshoots of Burhi Gandak. The Ganges also skirts the district on the south.

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

According to the 2011 census Samastipur district has a population of 4,254,782, roughly equal to the Republic of the Congo or the US state of Kentucky.This gives it a ranking of 45th in India (out of a total of 640).The district has a population density of 1,465 inhabitants per square kilometre (3,790/sq mi).Its population growth rate over the decade 2001-2011 was 25.33%.Samastipur has a sex ratio of 909 females for every 1000 males, and a literacy rate of 64.21%.

Culture of Samastipur is simple, beautiful and secular. It has an old tradition of folk songs and folk famous Mithila paintings, sung and done during festival and important social occasions like marriage and birth ceremonies. Several folk dance form also practiced as Kathghorwa nach , natua nach dom domin and jhijhia. Famous Sama Chakeva and Dom kach is also practiced as a culture in this area. Bhat dal and aaloo chokha is the happiest meal for every being & dhoti kurta for man & sari and salwar kurti for women is regular dress to wear.

The district has good education facilities. It has some reputed institutions such as A.N.D. College, Shahpur Patory, Samastipur College, BRB College, RNAR College. St. Kabir College. S M R C K college.Dr. LKVD College, BBN College. The district also boasts of a very reputed Agricultural College in Pusa, well known as Rajendra Agricultural University Pusa and a proposal of Government medical college by Government of india under Pradhan Mantri Swasthya Suraksha Yojana.