Jacqueline Veyrac, the 80-year-old owner of the five-star Grand Hotel in Cannes and the ritzy seafront La Reserve restaurant in nearby Nice, was snatched on the street on October 24 2016, and bundled into the back of a van.
She was held bound and gagged for two days in the vehicle, which was parked on a quiet street, before being spotted by a passer-by and released.
The main suspect in the trial, held in the southern city of Nice, is Giuseppe Serena, former manager of La Reserve who allegedly bore a fierce grudge against Veyrac after she terminated his contract in 2009.
Serena is accused of ordering her abduction with a view to obtaining a ransom with which he hoped to open a new restaurant.
His suspected accomplices have designated the bankrupt Italian, who is also accused of an earlier kidnap attempt on Veyrac in 2013, as the mastermind of the operation.
Serena, who has been in pre-trial custody for over four years, denies the charges.
All 13 accused were present in the courtroom at the trial's opening. Veyrac was expected to attend on Thursday when she is due to testify.
Back in 2016, two masked men pounced on Veyrac as she was about to get into her SUV and shoved her into a stolen van driven by an accomplice.
One of the kidnappers threatened to kill her if she made any noise.
She was also forced to drink a sedative.
Veyrac tried to call for help and bang on the doors of the van but only succeed in wriggling free and making contact with a passerby on her second night in captivity.
Former British soldier Philip Dutton, 52, has admitted involvement in the kidnapping.
Dutton, who is also accused over the 2013 kidnap plot, demanded a £4.5 million (€5-million) ransom for Veyrac of which he intended keeping 10 per cent, with 50 per cent meant for Serena and the rest for other accomplices, investigators said.
Among the other defendants are former paparazzi-turned-private-detective Luc Goursolas, who is accused of fitting a tracking device to Veyrac's car, and three suspected gang members from a poor Nice neighbourhood, accused of carrying out the kidnapping.
Veyrac's late husband developed the Grand Hotel in Cannes, one of the palatial seafront establishments favoured by the international jet set during the annual Cannes film festival.
