The improvised explosive device (IED) was detonated outside the gates of the seminary as a degree ceremony for students was being held inside, police told reporters.
Around five to six kilogrammes of explosive material was used in the bomb, police sources said.
The wounded were taken to Quetta's Civil Hospital, while 13 people who had serious injuries were shifted to Combined Military Hospital (CMH) hospital.
Emergency was imposed in all the hospitals of the city.
"Unknown men planted explosive material in a bicycle and parked it outside the gate of a religious seminary Jamia Islamia Miftahul Uloom, Satellite Town, Sariab," Deputy Inspector General (DIG) Operations Qazi Abdul Wahid was quoted by the Dawn news as saying.
The DIG operations said that the bicycle was loaded with flower garlands and the police personnel deployed for security outside the seminary and other people took it as they were brought for the students.
Quetta suffers from militancy linked to the Taliban, sectarian violence between the majority Sunni and minority Shia Muslim sects and a separatist insurgency.