The powerful collision also wounded 58 people and is being called the country's worst train disaster in more than 20 years.
Cars at the front of each train were turned into heaps of mangled metal while others were toppled on their sides.
One of the trains was on the wrong track. Maintenance work was being done on the tracks in that area, but officials say it's too early to say if that was the cause.
The U.S. consulate in Krakow, where the crash occurred, said an American woman was among the dead and her family had been informed. Spokesman Benjamin Ousley said he could give no more information and that Polish authorities would release further details at a later point.
Prime Minister Donald Tusk earlier had said that several of the passengers were foreigners, including people from Ukraine, Spain and France, but none of them were among the dead or mostly seriously injured.
The Associated Press contributed to this report.