"We have 15 dead" that authorities were in the process of identifying, said Aldo Tineo, a health official for the central city of Tarma.
The bus, which landed on its roof next to a river, had been transporting more than 60 people when it veered off course on a winding, narrow road, local media reported.
It had traveled about two-thirds of the way from the capital Lima to its destination, the city of La Merced.
Emergency teams were at the scene of the incident. The cause of the crash has not been identified yet.
Road accidents are a common occurrence in the South American country, due to a combination of speeding and poor road conditions, reports BSS.
Last year, a total of 3,173 traffic-related deaths were recorded, according to police.