The charge carries a maximum life sentence.
Around a dozen pirates attacked the luxury French yacht on April 4, some 850km off the Somali coast.
They then sailed the boat and its crew to Somalia and tried to flee inland after collecting the ransom.
French helicopters pursued one of their getaway cars and captured all those inside and retrieved some of the ransom.
Nicolas Sarkozy, the president of France, last week thanked the French army and other French agencies "that allowed a quick end" to the hostage taking, saying they were freed "without incident".