The explosion took place at the busy bus terminal in main Tirah bazaar, Kohat police chief Dilawar Bangash said.
Bangash said that a blast was suicidal, adding that the death toll is likely to rise.
Local police spokesman Fazal Naeem said the bomber approached the door of a bus carrying passengers and detonated his explosives.
Earlier it appeared the bomb was planted in a vehicle parked at the main bus stand, but Banghash said that "now we have found the head and legs of the suicide bomber".
