Dozens of other people were injured in the blast, which happened in the grounds of a government-run hospital in Quetta. The explosion took place shortly after the body of a prominent lawyer killed in a shooting attack was taken to the hospital.
At least 150 lawyers and journalists were gathered at the hospital when the bomb went off, police official Zahoor Ahmed said.
No-one has claimed responsibility for the attack.
It is not yet known who is behind the killing of the lawyer, Bilal Kasi, who was shot dead by two gunmen as he left his home for work.
In a statement Pakistan Prime Minister Nawaz Sharif expressed his "deep grief and anguish over the loss of precious human lives."
He said: "No one will be allowed to disturb peace in the province that has been restored due to countless sacrifices of security forces, police and the people of Baluchistan."
Sanaullah Zehri, the chief minister in Baluchistan province, said that the blast appears to be a suicide attack but that the police are still investigating.
The military has been deployed in and around the Quetta's hospitals.
Sky News Correspondent Neville Lazarus said: "I have been speaking to an analyst who said that the footprint of this blast is quite similar to those of the Taliban.
"They have done this in the past where they have attacked and then there has been a secondary device which has caused more casualties.
"This is similar to those attacks.
"There was one in 2013 where they killed a police officer and then a suicide bomber blew himself up amongst the mourners, killing at least 30 people."
The attack is the second deadliest bombing in Pakistan this year, after a blast in Lahore which killed 75 people in March.
At least 150 lawyers and journalists were gathered at the hospital when the bomb went off, police official Zahoor Ahmed said.
No-one has claimed responsibility for the attack.
It is not yet known who is behind the killing of the lawyer, Bilal Kasi, who was shot dead by two gunmen as he left his home for work.
In a statement Pakistan Prime Minister Nawaz Sharif expressed his "deep grief and anguish over the loss of precious human lives."
He said: "No one will be allowed to disturb peace in the province that has been restored due to countless sacrifices of security forces, police and the people of Baluchistan."
Sanaullah Zehri, the chief minister in Baluchistan province, said that the blast appears to be a suicide attack but that the police are still investigating.
The military has been deployed in and around the Quetta's hospitals.
Sky News Correspondent Neville Lazarus said: "I have been speaking to an analyst who said that the footprint of this blast is quite similar to those of the Taliban.
"They have done this in the past where they have attacked and then there has been a secondary device which has caused more casualties.
"This is similar to those attacks.
"There was one in 2013 where they killed a police officer and then a suicide bomber blew himself up amongst the mourners, killing at least 30 people."
The attack is the second deadliest bombing in Pakistan this year, after a blast in Lahore which killed 75 people in March.
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