They are charged with hijacking, kidnapping and armed robbery after allegedly seizing the boat and its crew, a married couple both aged 60.
French special forces stormed the yacht off Somalia, freeing the couple. A seventh suspected pirate was killed.
Defence lawyers say the suspects were forced to take part in the attack.
Pirates have seized scores of vessels, mainly cargo ships, off Somalia.
The defendants in Paris, aged between 21 and 35, face life in prison if convicted.
They are accused of attacking the Carre d’As on 2 September 2008 and demanding a ransom of $2m (£1.3m; 1.5m euros) for the release of French couple Jean-Yves and Bernadette Delanne.
The yacht and its owners were freed by the French military two weeks later.
A lawyer for one of the defendants said the unnamed accused man was a fisherman who had been forced to take part in the attack because he was a seaman.
Another lawyer said the organisers of the attack were still at large.
Somali suspects in three other French piracy cases are currently awaiting trial.
The International Maritime Bureau has said that better policing and improved security have reduced successful hijackings by Somali pirates this year.
Nevertheless, attacks linked to Somalia made up more than half the piracy incidents reported worldwide.
A US study found that maritime piracy costs the global economy $7bn-12bn a year.