The members of the Alizai tribe had met in the town of Ghaljo in mountainous Orakzai, which is the only one of the tribal regions that does not border Afghanistan.
A security official said that the attack came a day after tribesmen had targeted two hideouts belonging to pro-Taliban groups operating in the area.
"The tribesmen blew up two hideouts of the militants a day earlier and it is possible this attack was in revenge for their actions," a security official told the AFP news agency on condition of anonymity.
The bombing came a day after four tribal elders in Bajaur, a tribal region north of Orakzai, were abducted and beheaded after attending another pro-government meeting, officials said.
"People will tell you that Pakistan is already in a state of war. Every day there are suicide bombings," Al Jazeera's Kamal Hyder, reporting from the North West Frontier province, said.
"The violence is escalating at a time that the national assembly is not able to come to grips with the situation.
"The death toll could rise further," he said.
Violence has intensified across Pakistan in recent months since the army began an offensive against the pro-Taliban and al-Qaeda forces in the Bajaur and Swat regions.
Orakzai, near the main northwest city of Peshawar, has been relatively peaceful compared to the other tribal regions.