Justice Maqbool Baqar, senior Judge of Sindh High Court, was heading to the court when the bomb believed to be attached to a motorbike exploded near his motorcade.
At least 15 people, including Baqar, were also injured in the blast, a police officer said.
The injured include four police personnel and a paramilitary Rangers officer.
Baqar was shifted to a private hospital for treatment.
Police said that driver of the judge was among the dead.
"The bomb was apparently set off by a remote device and the explosion injured bystanders and damaged vehicles and bikes in the security convoy," Senior Superintendent of Police South Aamir said.
"The convoy included police mobiles and rangers and those killed include four security officials and the injured two females who were bystanders when the explosion took place," he said.
Sharjeel Memon, Information Minister in SindhProvince of which Karachi is the capital, confirmed that seven people died in the blast.
A police mobile van and two motorbikes were damaged in the blast.
Deputy inspector general Ami Shaikh said that six teams have been formed to investigate the attack.
No group claimed responsibility for the attack.
Taliban and banned militant groups are often blamed for such attacks.