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, which is celebrated on Sunday there.
The victims are believed to be mainly women and children.
Amric Cicek, governor of Mugla province, suggested the brakes may have stopped working.
But the mayor of Marmaris, Ali Acar, told Turkish newspaper Hurriyet: "I think that the accident was a result of driver error."
Turkey's Andalou news agency said the crash took place on the Mugla-Antalya road at the Sakar Gecidi pass.
The vehicle landed on another section of road below after it went through the crash barrier.
Rescue teams were dispatched and the injured were taken to hospital, Andalou said.

Trainers contributed £98.6m and stud farms £81.2m, while visitors bring in £8.8m to the racecourses.
Forest Heath District Council commissioned research company SQW to carry out the study into the impact of the sport on the 'Home of Horseracing'.
The SQW report did not try to measure the amount spent by owners and visitors at local shops and on services.
The council said the survey, commissioned along with the Newmarket Horsemen's Group (NHG), aimed to "understand the scale and economic significance" of the industry.
SQW report
William Gittus, chairman of the NHG, said: "It's a global industry and we are keen to attract global investors and to develop planning and economic policy.
"We who work in the industry know it's very valuable to the local economy, but we needed to provide concrete evidence to people who make those policy decisions."
Councillor James Waters, Conservative leader of the the Council, said: "We have never before had an independent, fact-based report on exactly what impact the horse racing industry has on Newmarket.
"It looks at the impact of racing up to 25 miles away, so will be a really useful document as we plan future growth in west Suffolk."
The survey did not attempt to calculate how much visitors or people in the racing industry spent in the town centre.
Trainer John Gosden said: "Obviously the report has been very careful to never have any figures challenged and I think they've underestimated the financial contribution to the region, but they didn't have access to people's private accounts.
"What we have had here in the last 15 years is an incredible investment from the Middle East and Far East."
Kevin Pearson, from Golding clothing shop and a member of the Newmarket Retailers Association, said: "The majority of horse owners and training staff do shop locally and this adds to the vibrancy of the high street."

The Welsh Rugby Union (WRU) are set to take over the Dragons as well as their Rodney Parade ground, but the deal will not be completed until 1 July.
75% of Newport RFC shareholders must still ratify the deal which chairman Martyn Hazell called for them to back.
"Until something gets sorted above, we've got to get on with our jobs," Evans told BBC Radio Wales.
He continued: "The future for Dragons rugby is positive and as players we've got to make sure it happens on the field and whatever goes on off the field is down to the bigwigs.
"Whatever goes on at the top, hopefully it moves in the right direction and we get a positive future."
Evans made his 200th appearance for the region in their 17-27 Pro12 defeat by Ulster and is proud of his record.
"They all merge into one and it's something I'm proud of. Running out [against Ulster] was really emotional," he said.
"It's nice to be able to come out in the last game of the season at Rodney Parade which I was really looking forward to.
"I felt good going into this game and unfortunately it didn't quite go our way and I thought the performance was there.
"We want to play attacking and attractive rugby and get some wins."
At least 23 people have died after a tourist bus fell from a cliff near the southern Turkey seaside resort of Marmaris.
[NEXT_CONCEPT]
The horse racing industry is worth £208m a year to the economy in and around Newmarket, a study has revealed.
[NEXT_CONCEPT]
Newport Gwent Dragons captain Lewis Evans says their players are uncertain about the future of the region.