The explosion struck a truck carrying civilians, most of them laborers, in a far-flung frontier village in Bajaur tribal district, one of Pakistan's seven semi-autonomous tribal regions.
"At least three people have been killed and 26 wounded; the roadside bomb exploded when the truck passed by," Mustafa Khan, a local government official told AFP.
Anwarul Haq, a senior local government official, confirmed the casualties, telling AFP that the injured have been taken to local hospitals.
No militant group has yet claimed responsibility for the attack.
Violence in Pakistan has declined in recent years following a series of military offensives against insurgents along the northwestern border.
But the remnants of militant groups are still able to carry out periodic bloody attacks, particularly in the northwest.