The hold-up took place on Thursday night near the French-Swiss border.
Robbers posing as plumbers kidnapped the daughter of a cash van driver and demanded the vehicle's contents as ransom, investigators said Friday.
Swiss police said the father was en route to the city of Lausanne when he was telephoned by the robbers who told him to leave the motorway and head for a carpark, where black-clad men made off with the money.
The driver's daughter was found unharmed on a road on the outskirts of Lyon later that evening.
No arrests have been made yet in the probe, which is being led by French police, while Swiss police have launched an appeal for witnesses to come forward.
The suspects are said to have accents from the south of France or north Africa.