The mineral-rich southwestern region borders Afghanistan and Iran and has been troubled for decades as Baloch insurgent groups fight against the state, saying it denies them their share of regional resources.
No group has claimed responsibility for the attack on Friday.
"A group of armed men attacked the Junaid Coal company mines in the Duki area in the wee hours using heavy weapons," said Humayun Khan, the police station house officer for the town, east of the city of Quetta.
They fired rockets and grenades at the mines as well, he said.
The attackers gathered the miners at one place and opened fire on them, local media reported, adding that they also set mining machinery on fire.
Ten coalmines were in the area, a company official told local media.
"We have received 20 bodies and six injured so far at the district hospital," said Johar Khan Shadizai, a doctor in Duki.
Pakistan has seen a resurgence of Islamist militancy since 2022 when a ceasefire between the Pakistani Taliban and the government broke down.
Two Chinese nationals working for a power plant were killed and a third injured in an explosion near the international airport in the southern city of Karachi earlier this week.
The Baloch Liberation Army (BLA), one of several insurgent groups battling the government, claimed responsibility for the attack.
BLA was also behind Balochistan's most widespread violence in years in August, when separatist militants attacked police stations, railway lines, and highways, killing more than 70 people.
The region saw violence last month too when armed men stormed a residence housing labourers from eastern Punjab province, killing seven of them.
The attacks come as the country is preparing to host the summit of the eight-member Shanghai Cooperation Organisation (SCO) in capital Islamabad next week.
The event is expected to see high-level Chinese representation and will also be attended by India's foreign minister.
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