The name "Dupax" comes from the Isinai word "dopaj". By "dopaj" the Isinais, who are the first inhabitants of Dupax, meant "to lie down in complete relaxation." According to folklores that have survived from generation to generation, even long before the founding fathers established the municipality, the site of what eventually became the "poblacion", or town proper of Dupax, used to serve as a roaring camp for primitive hunters from surrounding tribal settlements. After hard days of hunting in nearby mountains, the hunters would repair the camp where they would feast on their catch of wild animals. When they were through with their brand of merrymaking "they would lie down and relax completely" before getting up again to return to their respective home. The camping area, which was a plain or a valley, was near their hunting grounds that later, they decided to settle on it together with their families and the town of Dopaj was established.
In time, the town's name metamorphosed into Dupax, after the arrival of the Spaniards who, for convenience, substituted the letter "x" in lieu of letter "j" for easy pronunciation.
The original town of Dupax is the largest municipality of the province of Nueva Vizcaya in terms of land area. But in 1971, the Congress passed into law Republic Act 6372 otherwise known as "An Act Creating the Municipality of Dupax del Sur from the Municipality of Dupax in the province of Nueva Vizcaya", sponsored by Congressman Benjamin B. Perez in the Philippine House of Representatives and Senator Leonardo B. Perez in the Philippine Senate. PresidentFerdinand Marcos amended some sections and signed it into law with the promulgation of Presidential Decree 586 on November 26, 1974 which paved the way for the division of Dupax into two municipalities: Dupax del Norte and Dupax del Sur.