Reuters reported 17 dead, but the BBC said there were at least 23 victims.
Another 11 were injured when the driver lost control of the minibus and ploughed through a crash barrier.
Local media said no foreign tourists were among the passengers.
Turkish media said the vehicle was heading towards Marmaris for a mother's day event.
The victims are believed to be mainly women and children.
Amric Cicek, governor of Mugla province, suggested the brakes may have stopped working.