According to the national census conducted in April 2017 by the government, Thanh Hoa province had a total population of 4,291,000, becoming the third largest province in the country in terms of population after Ho Chi Minh City and Ha Noi. Population density was 305 people per square kilometer and gender rate was 98 boys per 100 girls. The province is home to many ethnic groups of Dao, Hmong, Kho Mu, Kinh, Muong, Thai, and Tho.
March 19, 1626, Jesuit Father Alexandre de Rhodes arrived at Cua Bang (now Ba Lang parish) in Thanh Hoa province and started to introduce Catholicism to people in this area and later in other places in northern Vietnam.
Thanh Hoa was carved out from the neighboring Phat Diem diocese and erected on May 7, 1932. When the Vietnamese Church Hierarchy was established in 1960, Thanh Hoa was elevated to a diocese.
After French troops were defeated by communists at Dien Bien Phu battlefield in 1954, French Bishop Louis de Cooman Hanh, the first bishop of the diocese, was forced to leave for France, and 18,500 local Catholics fled to the south. Only 27 priests, 85 seminarians, 50 nuns and 47,000 Catholics stayed in the diocese.
Father Peter Pham Tan was named bishop of the diocese on March 17, 1959 but his ordination was not until on June 22, 1975. He died in 1990.
After Vietnam adopted the open-door policy in 1980s, religious activities gradually resumed. Bishop Bartholomew Nguyen Son Lam, who led the diocese 1994-2003, rebuilt churches, bishop's house and convents and improved spiritual life of local Catholics.
In recent years the diocese built a minor seminary to train future priests, sent many priests and nuns to study abroad. The local Church plans to build facilities for orphans and people with physical disabilities in the future.
The diocese is a suffragan of Ha Noi archdiocese and its titular is Saint Joseph, March 19.
The province's annual average temperature falls around 23.3 degree Celsius. The highest temperature in summer is up to 40 degrees Celsius and drops to 5 degrees Celsius in winter. Its rainfall is 1,730-1,980 mm mainly concentrating in June to November.
In a land area of 11,158 square kilometers, the diocesan territory covers the civil province of Thanh Hoa, that borders Ninh Binh on the northeast, Son La on the northwest, Laos on the west, Nghe An on the south and East sea on the east.
In 2009, the province recorded 608,875 students from elementary to high schools .
Buddhism, Catholicism and Protestantism are main religions in the province.