The bus had been taking about 40 passengers to visit thermal springs when the accident happened in the Subang region of West Java province.
It appears the bus collided with a motorcycle as they went downhill, police quoted by local media say.
At least 16 people, including the driver, were injured.
Most passengers were from South Tangerang in Banten province, reports said. The bus had reportedly been on its way to hot springs near Mount Tangkuban Parahu on Saturday.
Subang police chief Budhy Hendratno said the victims had not yet been identified, the Wartakota news website reported. He said the exact cause of the accident was under investigation.
Serious road accidents are common in Indonesia.
Last July 10 people were killed when a bus travelling from Bali to a town in East Java hit the back of a lorry. (Courtesy BBC)