Disaster management officials said the death toll may increase.
Images on social media showed people in the city of Quetta out on the streets in the aftermath of the quake.
Initial reports indicated that it may have measured up to 5.7 on the Richter scale. A government official said people died when structures collapsed.
Local officials told BBC Urdu that at least 150 others are thought to have been injured, with several of them rushed to hospital in critical condition.
The Reuters news agency said some people were treated on stretchers with medics using phones as torches.
Rescue operations are still underway.
Many of the dead were women and children, according to Reuters.
More than 100 mud houses have collapsed and a large number of buildings were damaged, leaving hundreds of people homeless, an official told the agency.
Much of the damage appears to have affected the Harnai district, officials said.
Harnai, which is located east of the provincial capital Quetta, has a large number of coal mines.
Balochistan's interior minister Mir Ziaullah Langu told the BBC that emergency services had been dispatched to the area.