There were no reports of casualties, but state broadcaster TRT Haber said Yalova and Kumkoy villages, close to where the fire broke out, were being evacuated. It was not clear how the fire started or over how large an area it had spread.
Strong winds fanned the fire and a huge pall of smoke hung over the southern part of the peninsula as fire raged below among the trees, state television footage showed.
Fire fighting teams were using helicopters to drop water and Forestry Minister Bekir Pakdemirli was travelling to the area, TRT said.
Pakdemirli was cited as saying about 400 people were working to fight the blaze, using 107 fire trucks, two planes and 20 helicopters.
The peninsula attracts many visitors each year to pay homage to those who fought during the Gallipoli campaign of World War One. The blaze was not near the cemeteries built for the war dead.