Reports suggest some of those injured are in serious condition.
It is the latest in a series of at least six recent suicide attacks in Pakistan that have put security authorities on high alert.
City police chief Rauf Khan said the blast at the city's district-court complex killed a judge and several lawyers, as well as relatives of prisoners on trial.
No one has claimed responsibility.
Quetta is the capital of Pakistan's restive Baluchistan Province, which has witnessed several terror attacks in recent months.
Authorities have blamed a number of attacks on Taliban militants fighting security forces in Waziristan's tribal belt, which borders Afghanistan.
(AFP, Reuters, AP)