The bus was taking passengers to the central city of Pichanaki yesterday when the driver lost control and ran off the road, falling 35 meters into the Tarma river, said Jose Mendoza, police spokesman for the region of Junin.
The accident totalled the bus and left another 10 people injured.
Initial investigations indicate the driver was speeding and lost control on a curve, Mendoza said.
Fatal road accidents are common in Peru, in part because of lax traffic enforcement but also because of the Andean country's challenging mountain terrain.