A judge was among the dead and the court was said to be in session when the device exploded on Saturday.
Many of the dead are lawyers.
Rehmat Niazi, the police officer in charge of operations, told Reuters news agency: "Twelve people, including a senior judge and six lawyers, have been killed in the blast and over 20 people wounded."
The court is near to a police station and an office that issues driving licences.
Güncelleme Tarihi: 20 Eylül 2018, 18:16
There has been a spate of suicide attacks in Pakistan that intelligence officials have linked to groups operating from tribal areas, seen as supporting the Taliban and its battle against Nato forces in Afghanistan.
It was not clear who was behind the blast.
Quetta is the capital of Baluchistan province which has seen ongoing unrest. Residents of the province have been demanding a greater share of wealth from their natural resources.
Police found a head among the damage, raising suspicions that the blast could have been carried out by a suicide bomber, Niazi said.
Lawyers and officials surround wounded
The attacks followed a Pakistan army air strike on a base in Waziristan in mid-January.
Pakistan has been under mounting pressure from the US and Afghanistan to tackle Taliban sanctuaries on its territory.
Taliban leaders are widely believed to be operating from in and around Quetta, though Pakistan consistently denies their presence.