Ms Pieau was arrested earlier this month. An unnamed 28-year-old Paris agency photographer has also been arrested and charged, while another is "on the verge" of being charged.
Publishing director Ernesto Mauri was charged in April alongside a photographer believed to be Valerie Suau, who admits taking pictures of Kate in the south of France last September but says her photographs were "all decent". However, according to the Telegraph, she is suspected of assisting at least one other photographer who took snaps of the topless Duchess.
The photos in question were taken while Kate, 31, was sunbathing with her husband Prince William at Chateau d'Autet in Provence, France. During a hearing in September, a lawyer representing the couple said the pictures were a "grotesque breach of privacy".
After deciding to publish the photos, Ms Pieau told Agence France-Press that she had no regrets. "These photos are not in the least shocking," she said at the time.
"They show a young woman sunbathing topless, like the millions of women you see on beaches.
"It is a young couple that has just been married," she continued. "They are in love. They are beautiful. She is the princess of he 21st century."