It was found at the Ethnic Boarding High School in Dien Bien Phu Town last Saturday by workers building an anti-erosion embankment for the school.
The rusted 350 kg bomb lay two meters under the ground.
The province Military Command said it had been dropped by the French during the Battle of Dien Bien Phu, part of the First Indochina War that Vietnam fought between 1946 and 1954.
The bomb was taken to a designated site for detonation.
Since 1975, when the war against the U.S. ended, there have been more than 40,000 deaths and 60,000 injuries due to unexploded ordnance (UXO).
According to the Vietnam National Mine Action Center, all 63 provinces and cities have UXO spread over some 5.6 million hectares of land, or 17.71% of the country's area.
It is estimated that 600,000 – 800,000 tons of UXO still lie below ground.