The attack took place in Lakki Marwat, a district in Khyber Pakhtunkhwa province, local police chief Azmat Ullah said on Tuesday.
He said two traffic police officers and a woman were among those killed.
No group immediately claimed responsibility for the bombing, but suspicion is likely to fall on the Pakistani Taliban, known as Tehrik-e-Taliban Pakistan, or TTP.
The group is separate from but allied with Afghanistan's Taliban government and has intensified its campaign against Pakistani security forces in recent years.
The latest attack came days after 15 police officers were killed in a suicide bombing and gun assault on a security post in the nearby Bannu district, prompting Islamabad to summon a senior Afghan diplomat to lodge a formal complaint.
Pakistan blamed the late Saturday attack on the TTP.
Pakistani authorities have long accused Afghanistan's Taliban government of sheltering TTP militants.
Kabul has denied the allegation, saying it doesn't allow militants to use Afghan soil to launch attacks against other countries.
Pakistan has witnessed a surge in militant violence in recent years, straining relations with Afghanistan.
The TTP and other militant groups have grown more emboldened since the Afghan Taliban returned to power in Kabul in 2021.
Tensions between Pakistan and Afghanistan have persisted, including fighting that has killed hundreds of people since late February.
In early April, the two sides held peace talks mediated by China.
However, sporadic cross-border clashes have continued, though at a lower intensity than before.
