Somali suspects in three other cases are currently awaiting trial.
The men face charges of hijacking, kidnapping and armed robbery and could receive life in prison if found guilty.
The six were captured and flown to France after French special forces stormed the yacht, the Carre d’As IV and rescued Jean-Yves Delanne and his wife Bernadette. A seventh suspect was killed in the raid. They had reportedly demanded a ransom of two million euros for the couple’s release.
Judges are expected to close the hearing to the public because one of the men was a minor at the time of the crime.
Kidnappings off Somalia’s 3,700 kilometres coastline have become more common with dozens of ships seized in recent years.
Ships are often held for weeks until they are released for large ransoms usually paid by governments or owners.