Caroline Chassain, spokeswoman for the Nanterre prosecutor, said that Mondadori Magazines France and local photographer Valerie Suau were placed under investigation earlier this month over possible criminal exploitation of the images, which appeared in the French "Closer" magazine last September.
The photos showed the Duchess of Cambridge relaxing at a private villa in Provence, in southern France, sometimes without her bikini top and, in one case, her suit bottom partially pulled down to apply sunscreen.
Chassain added that Suau's employer, local newspaper "La Provence," was also placed under formal criminal investigation on Monday, but she did not elaborate.
The investigation will likely take months to complete.
The blurry photos have been called a "grotesque and totally unjustifiable" abuse of privacy by British royal officials. Despite efforts by Prince William and Kate to halt their use last September they went on to be published in Italy, Ireland, Sweden and on the Internet. However, no major British publication carried the photos, including Rupert Murdoch's top-selling U.K. tabloid The Sun, which last year ran photos of a naked Prince Harry cavorting in a Las Vegas hotel room.
The first major press incident involving William and Kate brought back memories of William's mother Diana being hounded by paparazzi in France in the hours and days before her fatal car crash there in 1997.