Framarz Sikandar, an official with the governor's office in the western province of Hirat, said the bus was travelling fast on a narrow road and the driver apparently lost control.
Among the dead are several children.
The same narrow stretch of road was the site of a similar bus crash last month that killed 22 people.
Elsewhere in Afghanistan, a roadside bomb killed two people and wounded three others in Helmand province.
Omar Zwak, a spokesman for the provincial governor, said the bomb detonated as a civilian car containing members of a wedding party was passing.