The blast, like the earlier ones, took place close to the border regions with Afghanistan, the stronghold of Islamist militants threatening the stability of nuclear-armed Pakistan.
The bomber destroyed a small passenger bus at a station in a market area of Kohat city, photographs taken by The Associated Press showed. Many of the victims were onboard the vehicle. It was unclear whether the attacker was inside the bus.
Al-Qaida and Taliban militants seeking to overthrow Pakistan's broadly pro-Western government have carried out scores of bombings in recent years, mostly against Western, government or security targets. The army has responded with offensives in some parts of the northwest.
Most of the buses at the stand were heading to Orakzai, a border region that has seen army operations against the militants this year. Shad Ali, a doctor who heads the health administration in Kohat, said 15 were killed and 25 injured in the blast.
The attack occurred on the first day of the Islamic month of muharram, which often sees attacks by Sunni extremist groups on minority Shia Muslims. Orakzai and Kohat city are home to some Shia, but the religious affiliations of the victims were not immediately known.
On Monday, 50 people were killed in two blasts at a government compound in Mohmand, also in the border area. A spokesman for the Pakistan Taliban, the main militant group in the northwest, claimed responsibility for that attack earlier Wednesday.
On Tuesday, a suicide bomber attempted to kill the chief minister in the Baluchistan province, which also borders Afghanistan.