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.
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Sharjeel Memon, Information Minister in Sindh Province of which Karachi is the capital, confirmed 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.