The blast occurred at Kamrani Road in Quetta, the capital of the restive Balochistan province.
Police officials said unidentified persons had planted the bomb along the roadside and detonated it by remote control when a police vehicle was passed through the area.
Three policemen and a civilian were wounded by the explosion. They were taken to a nearby hospital, where a policeman succumbed to his injuries, officials said.
The attackers managed to escape.
Law enforcement personnel cordoned off the area and launched a search operation.
No group claimed responsibility for the attack though such bombings are usually blamed on Baloch nationalist groups.