The bus was traveling from the resort of Alanya to Pamukkale when it left the road and plunged into the Aksu River about 50 feet below the highway, Russian news agency RIA Novosti reported.
The Russian consul in Antalya said 25 Russians were injured. The bus was carrying mostly Russian tourists, the diplomatic official said.
Preliminary investigations suggest the driver fell asleep, RIA Novosti said.
The Turkish NTV channel said the bus crashed into a roadside barrier, carrying it to the bottom of the ravine, where the barrier "turned into a pile of metal."