The crash occurred in the early hours of Saturday morning on the A4 motorway which links Poland and Germany.
The Polish coach smashed into a Ukrainian bus in front of it, skidding across the motorway and colliding with a Polish minibus, which was travelling in the other direction.
All nine people sitting in the minibus died, along with two people on the Polish coach, police said in a statement. All the victims were Polish nationals.
Prosecutors opened a preliminary investigation against the 44-year-old driver, who was also injured in the crash, on suspicion of manslaughter due to negligence.
An inspection of the three wrecks is expected to be completed this week, a police spokesman said.