Governor Muammer Guler, quoted by the state Anatolian news agency on Monday, said one of the six was seriously wounded. Police had said earlier that the blast had killed at least one person but the governor, visiting the scene, made no mention of any deaths.
"Our teams are carrying out an investigation. There is explosive material. We will soon determine what kind of material it is," Guler was quoted as saying.
Asked if he thought the blast was a terrorist attack, Guler said: "Everything is possible."
Last Thursday, a bomb explosion at an Internet cafe in Istanbul killed one person and wounded 16, an attack claimed by a militant Kurdish group.
The group, called the Kurdistan Liberation Hawks, is believed to have links to outlawed Kurdish rebels fighting security forces in southeast Turkey and has claimed responsibility for other attacks in the past.
CNN Turk television showed wounded shoppers being helped into ambulances after Monday's 7.30 pm (local time) blast. The ground was littered with glass from blown-out windows.
The store belongs to the Kiler chain and is in the Bahcelievler district near the airport on the European side of Istanbul, a sprawling city of 12 million people.
Anatolian quoted local police chief Celalettin Cerrah as saying the blast had occurred near the supermarket entrance at a place where customers can leave bags before doing their shopping.
Cerrah said the supermarket lacked security cameras. Islamists, ultra-leftists and other militant groups have attacked civilian and military targets across Turkey in the past.
Islamists set off a series of devastating bombs in Istanbul in November 2003, targeting British and Jewish sites and killing more than 60 people.