In a land area of 16,089 square kilometers, the diocese is located in the northern-western part of Bihar state having its headquarters in Bettiah which is also the headquarters of the civil district of West Champaran. The diocese shares a 100-kilometer-long border with Nepal in the north. It borders the Diocese of Muzaffarpur in the east, the Diocese of Gorakhpur of Uttar Pradesh state in the west and the river Ganges in the south that separates it from the Archdiocese of Patna.

The diocesan territory includes the five civil districts of West Champaran, East Champaran, Gopalganj, Saran and Siwan, and includes major towns such as Bettiah, Bagaha, Ramnagar, Narkatiaganj, Motihari, Sugauli, Raxaul, Areraj, Gopalganj, Siwan, Chapra and Sonepur.

The total population of the diocese is 15,086,337 comprising 7,732,000 males and 7,354,337 females. There are two tribal groups called Tharu (Mongoloid) and Oraons (Austric Dravidian); two semi-tribal groups called Musahar (Austric) and Chero (Austric Dravidian); and non-tribal groups comprising Hindus, Muslims and Catholics.

The diocese had its beginnings in 1703 when the Vatican's Congregation for the Doctrine of the Faith erected the prefecture of Tibet-Hindustan, entrusting it to the Capuchin Fathers of the Italian Province of Picenum in the Marches of Ancona. Father John Francis of Camerion, OFM Cap, was made prefect.

One of the Capuchin missioners, Father Joseph Mary, happened to spend some time in Bettiah in 1740 en route to Tibet and managed to cure the queen of Bettiah of a serious malady. The King of Bettiah, Dhruva Singh, asked the priest to stay in his kingdom, but the priest expressed his inability to do so unless the Vatican granted permission.

Subsequently, King Dhruva Singh wrote two letters to Pope Benedict the XIV requesting that the missioners be allowed to open a station in Bettiah. The pope replied to him on May 1, 1742, that he was allowing Capuchins to stay and preach the Gospel in the kingdom. Father Mary thus founded the Bettiah Mission in 1745.

In 1820, the Vatican elevated the Prefecture of Tibet-Hindustan to a vicariate with its headquarters at Agra, currently located in the northern Indian state of Uttar Pradesh. In 1827, an independent Patna Vicariate comprising Bettiah, Chuhari, Patna City, Danapore, Bhagalpur, Darjeeling, Sikkim, Nepal and some other adjacent territories was constituted by a papal decree. Father Anastasius Hartmann, OFM Cap, was nominated its vicar and ordained Bishop of Agra.

Patna Vicariate's first diocesan priest, Father Cajetan Casary of Bettiah, was ordained a priest in Rome in 1861.

In May 1892, the Vatican formed the new Prefecture of Bettiah-Nepal which was dissolved in 1919. The territory was then assigned to the Diocese of Patna under its first bishop, Louis Van Hoeck, SJ, of Ranchi Mission. In March 1980, the Patna diocese was bifurcated into the Patna and Muzaffarpur dioceses. In August 1998, the Muzaffarpur diocese was further divided into Muzaffarpur and Bettiah dioceses. Diocesan Father Victor Henry Thakur of Raipur diocese in Chatisgarh state was the first Bishop of Bettiah.

The per capita income in the diocesan territory is 4,097 rupees (about US$90). Sugar cane, saw and rice mills constitute the major industries. Sugar cane and paddy are the major agricultural products. The diocese has the most luxuriant forests in the entire state of Bihar. The diocese has about half a dozen perennial rivers and hundreds of natural ponds and lakes. Fishery is a major occupation.

Bhojpuri and Hindi are the major languages used in the diocesan territory. But English, Urdu and Kuduk (Oraon) languages are also used. The literacy rate in the area is 37.4 percent.