Security forces have been battling a years-long insurgency by militants in Balochistan demanding a bigger share of the province's wealth, as well as attacks by the Pakistan Taliban (TTP).
"The suicide bomber was riding a motorbike and hit the truck from behind," senior police official Abdul Hai Aamir told AFP.
The incident took place near Dhadar, the main town of Kachhi district, about 120 kilometres (75 miles) southeast of Quetta in Balochistan.
Photos of the aftermath showed the police truck upside down on the road with its windows shattered.
Mehmood Notezai, police chief for Kachhi district, told AFP the officers were returning from a week-long cattle show where they had been providing security.
There has been no claim of responsiblity for the attack.
"Terrorism in Balochistan is part of a nefarious agenda to destabilise the country," Prime Minister Shehbaz Sharif said in a statement released by his office.
The country is facing overlapping political, economic and environmental crises, as well as a worsening security situation.
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