There were around 100 people on board when two carriages left the tracks between the towns of Riedlingen and Munderkingen, near the corner of Germany that borders France and Switzerland, police in the city of Stuttgart said, Reuters reported.
The train was on a roughly 90 km (55 miles) route between Sigmaringen and Ulm. The cause of the crash was under investigation, the police statement said.
A picture by German news agency DPA showed carriages largely intact but jackknifed into each other and rolled onto their sides.
German national rail operator Deutsche Bahn (DBN.UL) said in a statement there were "many injured" and its thoughts were with the victims and their loved ones. It was not clear what had caused the train to derail, it said, adding that it would support the authorities in their investigation.