The attacker blew himself when the prayers was in progress at the mosque in the Anbar tehsil of the agency bordering Afghanistan.
"It was a suicide blast," a local official said.
At least 16 people were killed in the attack, local media reported, citing officials.
"Many people were gathered inside the mosque when a suicide bomber blew himself up," an eyewitness said.
Rescue teams and police rushed to the spot. The bodies and the injured are being shifted to local hospitals for medical treatment.
There was no immediate claim of responsibility, but Pakistani Taliban routinely targets courts, schools and mosques.
The attack came on a day when Prime Minister Nawaz Sharif vowed to continue the war against militancy and terrorism till elimination of the last terrorist.
During a meeting with Army chief General Raheel Sharif today, the prime minister expressed the resolve to continue the war against terrorism and militancy.
The army had launched operation 'Zarb-e-Azb' in June 2014 to flush out militant bases in the northwestern tribal areas.
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