Thirty people were injured in the crash, which occurred shortly after 7 a.m. local time (0500 GMT) near the town of Stammbach, around 90 km (56 miles) northeast of Nuremberg, police said. Some were seriously injured.
"It's clear now that all 18 of the missing people on the bus died in the accident," police said on Twitter.
There were 48 people, mainly from the eastern state of Saxony, on the bus and they were aged between 41 and 81, police said.
Speaking at the scene of the crash, Transport Minister Alexander Dobrindt said two people were in a critical condition, he said.
"The heat's development must have been intense because there is nothing flammable left on the bus . Only steel parts are recognizable so you can understand what that meant for the people in this bus ," he said.
Chancellor Angela Merkel described the crash as "terrible" and said: "Our thoughts are with the victims' relatives and we wish all of the injured a quick recovery, from the bottom of our hearts."