The bus was heading south with 25 people on board when the accident happened on the A2 highway at Sigirino, near Lugano — along a major route across the Alps from northern Europe to Italy.
It wasn't immediately clear what caused the bus to plow into the post, wrecking the front of the vehicle.
The bus had set off from the German city of Cologne on Saturday evening and was headed for Assisi in Italy, Johannes Huebner of bus travel group RDA, who was coordinating emergency plans for the company involved, told the German news agency dpa.
A group of young people and two drivers were on board, according to Ticino canton (state) police.
A police statement late Sunday said a 27-year-old German woman died of injuries sustained in the crash.
The statement said two people had serious but not life-threatening injuries, eight more had "medium" injuries and another four were lightly hurt.