The bomb detonated in the suburb of Kucukcekmece as the bus was passing a hospital, the World Bulletin reported.
"A bomb was placed by the side of the road and was exploded as the minibus passed," police spokesman Huseyin Capkin said, adding, "Fifteen of our friends are lightly wounded. We are investigating the incident."
It was not immediately clear who was responsible for the attack.
Istanbul hosted a regional summit Tuesday for leaders from Turkey, Russia and Iran, but the bombing happened more than 12 miles from the summit's location.