In a land area of 1,637.7 square kilometers, the diocesan territory covers the whole Nam Dinh province comprising Nam Dinh city and nine districts. Bui Chu is 100 kilometers southeast of Ha Noi.
The population in the diocesan territory is 1,218,100 at end of 2015. Most residents are Kinh or majority Vietnamese, and ethnic groups of Chinese and Tay.
Vietnamese is mainly spoken in the region.
The diocese is reportedly the country's first place where Catholicism was introduced by foreign missioners in 1533. It is also the cradle of Catholicism in the country. French Bishop Pierre Lambert de la Motte, one of the first two bishops of the Catholic Church in Vietnam, founded the first indigenous congregation of Adorers of the Holy Cross in the diocese in 1670.
Among 117 Vietnamese Martyrs, 26 martyrs were born in the diocese and 18 others were killed for their faith while serving the diocese that was home to execution grounds in 18th-19th centuries.
Trung vicariate was carved out of Dong Dang Ngoai vicariate in 1848 and served 139,000 Catholics. It was named Bui Chu vicariate in 1924. Bishop Dominic Ho Ngoc Can (1936-1948) was the first Vietnamese bishop of the vicariate. He founded the indigenous congregation of Our Lady of the Rosary in 1946 and many lay associations.
After French troops were defeated by northern communists in 1954, 142 priests and 100,000 Catholics fled to the south. Only 35 old priests, 54 preachers and 90 nuns remained in the vicariate.
The vicariate was elevated to Hai Phong diocese in 1960 after the Vietnamese Catholic hierarchy was established on Nov. 24 that year.
Many local seminarians were secretly sent to study theology and philosophy at Church-run institutes based in Ho Chi Minh City in the 1980s and 1990s. 20 of them were ordained priests without government permission. They quietly served local parishes without resident priests. They were officially regularized by Bishop Joseph Hoang Van Tiem after they finished a refresher course on theology at Nha Trang-based Major Seminary in 2006. In the following year, Bishop Tiem publicly ordained 61 priests. He also sends many priests and Religious to study abroad.
Bishop Tiem rebuilt the Bishop's House, built the diocese's pastoral center and Ave Maria Pilgrimage center, and repaired many old churches.
The diocese also creates jobs for local young people so as to prevent them from going to work in cities, and give scholarships to poor students. It builds good relations with followers of other faiths through charitable activities.
The local Church also plans to establish a diocesan major seminary in the near future.
The region has a monsoonal climate. The mean annual temperature is 23-24 degrees Celsius with July being the hottest month, averaging 29 degrees Celsius, and December and January, the coldest months with 16-17 degrees Celsius.
The mean annual precipitation is 1,750-1,800 millimeters. The region has two seasons. Rainy seasons last from May to October and dry seasons last November-April.
Lying in the northern gulf, the region is annually hit and affected by 4-6 typhoons.