Jamaat-ul-Ahra, a splinter group of the Pakistani Taliban, has claimed responsibility for the bombing that also injured 34 people, according to the group's spokesman Ehsanullah Ehsan.
The attack took place at one of the biggest mosques in Mohmand Agency, a district in Pakistan's Federally Administered Tribal Areas north of Peshawar, said Naveed Akbar, assistant administrator for the agency.
Two children under the age of 10 were killed in the blast while four children under the age of 10 are among the injured, Akbar said.
The official said the suicide bomber detonated his device inside the packed hall of the mosque, which has a capacity of 100 to 150 people.
Screams could be heard after the blast, and body parts were found scattered around the mosque, Akhbar said. He said the death toll is likely to grow.
Pakistani Prime Minister Nawaz Sharif strongly condemned the attack in a statement.
"The cowardly attacks by terrorists cannot shatter the government's resolve to eliminate terrorism from the country," he said.
Jamaat-ul-Ahra has carried out several major attacks in Pakistan this year, including a bombing on August 8 that targeted lawyers and journalists and killed more than 72 people.