The incident occurred in Sepoy village of Orakzai Agency, officials of the local political administration said.
The bomb was triggered by remote control while the bus was passing through the area.
Seven persons were killed instantly and six more succumbed to their injuries in a nearby hospital.
Two women, a three-year-old girl and an 11-year-old boy were among the dead.
The injured were taken to a hospital at Kohat in Khyber- Pakhtunkhwa province and officials described their condition as critical.
No group claimed responsibility for the attack.
Security forces and a local 'lashkar' or anti-Taliban militia launched a search operation after the blast.