Dawn newspaper reported the explosion occurred as the convoy of Sindh high court Justice Maqbool Baqir was traveling on Burns Road in the port city. Karachi is Pakistan's largest city and the capital of Sindh province.
The report said the banned Pakistan Taliban claimed responsibility. Its spokesman Ehsanullah Ehsan was quoted as saying his group targeted the judge for his "anti-Taliban and anti-mujahedin decisions."
Those killed included seven policemen, a paramilitary ranger and the judge's driver, the report said.
Baqir was critically injured and was moved to a private hospital where he was to undergo surgery, the report said.