It was not immediately clear what caused Saturday's blast, which came after a bomb attack last week in the nearby resort of Cesme, in which more than 20 people were injured, including three foreign tourists.
Separatist Kurdish rebels claimed responsibility for that blast.
The minubus was one of the local transport vehicles shuttling between downtown Kusadasi and a nearby beach, the Anatolia news agency said.
The resort is popular with foreign tourists.
Kusadasi, 72km (45 miles) southeast of the port city of Izmir, was hit by a bomb in April, which killed a police officer and wounded four people.
Kurdish separatists claimed the attack and vowed to carry out more in tourist areas.
The coastline is popular as a
summer tourist destination
The rebel group, considered a terrorist organisation by the United States and the European Union, says revenues from Turkey's lucrative tourism industry help finance the country's military.
Militants of the Kurdistan Workers' Party have battled government forces since 1984 in a conflict that has killed more than 37,000 people in southeastern Turkey.
Fighting in the region tapered off after a truce in 1999, which followed the capture of leader Abdullah Ocalan. But there has been a surge in violence since June when the rebels declared an end to the ceasefire, saying Turkey had not responded in kind.