Samastipur is a district of Bihar state with its head quarters located at Samastipur town. Earlier, Darbhanga was ruled by the Oinwaras (1325-1525 AD.), also often called as the Kameshwara Thakur or Sugauna Dynasty. At that time the whole Mithila region was under the Muslim rulers. The Oinwara Dynasty is well known for its support of leaning and fine arts. The period witnessed of some famous scholars-gadadhara, Sankara, Vachaspati Mishra, Vidyapati, Amartakara and Amiykara. Kameshwara, the founder of dynasty, was resident of village Oini, near Pusa Road, in the Darbhanga district. At that time of Hazi Ilyas of Bengal divided the Torhut region into two parts namely the southern part of the district was under Hazi Ilyas whereas the northern part of the district under Oinwaras. The original name of the Smastipur district was Shamsuddinpur, which was founded by Hazi Shamsuddin Ilyas. In the year 1976, Samastipur was emerged as a separate district in the map of Bihar, when it was split from Darbhanga district.

Shilpa Singh, a prominent winner of I Am She 2012, an Indian Beauty  pageant was born in 1990 at Samastipur. She was also among the top 16 contestants of Miss Universe 2012 pageant at Las Vegas in which she represented India.

Geographically, the district lies at 25°86'N latitude, 85°78'E longitude and 36 m Altitude. In the year 2021, there was a total 5.31% forest area of total geographical area. The district occupies a geographical area of 2,904 sq. kms. and it is bounded by Muzaffarpur district on the North West, Darbhanga district on the North East and East, Patna district, Begusarai district and Khagaria district on the South and Vaishali district on the West. Numerous rivers that passes through the district are Budhi Gandak, Baya, Kosi, Kamla, Kareh and Jhamwari and Balan, which are both the offshoots of Burhi Gandak. Natives experience extreme hot during the summers. Most of its rainfall occurs in the monsoon season. The actual rainfall in the district was 1655.5 mm in the year of 2021-22.

Administration wise, the district is divided into 4 sub-divisions Dalsinghsarai, Patori, Rosera and Samastipur Sadar. There are 2 Lok Sabha and 10 Assembly constituencies in the district. Moreover, the district comprises 7 towns and 1246 villages. The administrative language in the district is Hindi.

According to 2011 census the district has a population of 42,61,566 including 22,30,003 are males and 20,31,563 are females. The district has a sex ratio of 945 females for every 1000 males. During the year 2001-2011 the population growth rate in the district was 25.53% out of which 26.65% were males and 24.32% were females. As per 2011 census the major religions in the district are Hindu and Muslim with 89.18% and 10.62% of the total population. The population density in the district is 1,467 persons per sq. km. According to 2011 census the principal languages in the district are Hindi and Maithili with 60.74% and 30.79%. In the year 2020 the number of live births in the district was 1,26,188 out of which 67,950 were males and 58,224 were females. In the same year the number of deaths in the district was 15,387 out of which 9,562 were males and 5,819 were females.

The economy of the district is mainly dependent on agriculture. Since its soil is very fertile so the district is rich in almost all agricultural products. The chief agricultural crops in the district are paddy, maize, litchi, mangoes, potatoes, tobacco, chilli, turmeric, etc. The district has some small industries such as sugar mill, paper mill, jute mill and railway factory. A huge chunk of revenue comes from these small industries every year. In 2006 the ministry of Panchayati Raj declared it as one of the country's 250 most backward districts and it is currently receiving funds from the Backward Regions Grant Fund Programme (BRGF). In the year 2011-12 the gross domestic product in the district was Rs. 6,95,256 lakh at current price and Rs. 4,20,763 lakh at constant prices in the year 2004-05. In the year 2011-12 the net domestic product n the district was Rs. 6,26,244 lakh at current price and Rs. 3,73,954 at constant prices in the year 2004-05.

Lalit Narayan Mithila University, Darbhanga plays a vital role in the district’s education since most of its colleges are affiliated to Lalit Narayan Mithila University. IGNOU has set up its several educational centres in the district. The Rajendra Agricultural University is one of the renowned universities established near the Samastipur town, in Pusa. As per 2011 census the literacy rate in the district is 61.86% including 71.25% are males and 51.51% are females. The number of literates in the district is 21,42,880 including 12,93,575 are males and 8,49,305 are females.

Karpoori Thakur, an eminent Indian politician and the 11th chief minister of Bihar from December 1970 to June 1971 as a member of Socialist Party/Bharatiya Kranti Dal and from December 1977 to April 1979 as a member of Janata Party, was born on 24th January, 1924 at Pitaunjhia present day’s Karpuri Gram in the district. He was popularly known as Jan Nayak meaning the people’s leader. He also served as the deputy chief minister of Bihar from 5th March, 1967 to 31st January, 1968.

The district has some temple which allures numerous pilgrims from across the country. Mannipur Temple of Samastipur town has been devoted to the Goddess Durga. People from far distance place come to this temple to offer their prayers. The Thaneshwar temple, situated in the centre of the town is dedicated to Shiva, Rama, Ganesha, Hanuman, Shani and Kali. Khatu Shyam Mandir, a temple of Lord Krishna is located in the district. During the occasion of Janmashtami travellers from different regions come to this temple to worship. Baba Khudneshwar Dham, 16 kms away from Samastipur is a temple of Lord Shiva. This Dham depicts the picture of Hindu-Muslim unity because within a distance of about 3 meters there is Majar of Khudno Bibi along with Sivling.