More than a dozen people were wounded in the bomb attack in Faisal Avenue,
one of the main roads in Pakistan's economic capital, which Nato uses to ship supplies to troops in Afghanistan.
A passing motorcyclist was also killed in the blast. Four others were killed on Tuesday when two navy buses were attacked.
Authorities blame organisations linked to the Taliban and Al-Qaeda, even though, only last week, Pakistan's army chief General Ashfaq Kayani claimed his forces had "broken the back of Taliban and Al-Qaeda militants".
The bomb exploded close to the scene of Pakistan's deadliest attack, which killed 139 people at the 2007 homecoming of the former prime minister Benazir Bhutto, who was subsequently killed in a separate bombing two months later.