Food and drink firms must use one of four health messages on all broadcast and print adverts for their products.
Failure to do so could see them fined 1.5 per cent of their advertising budget.
The move is France's answer to the same obesity and junk food problems causing increasing concern in many western nations.
Messages that can appear on ads are: "Avoid snacking between meals" , "Avoid eating too much salt, sugar or fat" , "take regular exercise" and " eat at least five portions of fruit and vegetables every day" .
In true bureaucratic French style, the law also states that messages on televised and print adverts must take up at least seven per cent of the screen or page.
On radio, the message must be read aloud immediately after the advert.
Around 12 per cent of French people were obese and 29 per cent overweight in 2006, according to official figures.
Children have been increasingly affected, with nearly fifth of those under eight years old certified as obese.
The new advert rules gained muted support from France's food and drink industry association, Association Nationale des Industries Alimentaires (ANIA) , which had feared stricter measures along the lines of the UK's ban on junk food advertising to children.
The group said it was " engaging strongly [with government] on all fronts" to ensure consumers had a balanced diet.
ANIA president Alain Bazot was unavailable for comment Thursday morning.
Most opposition to the new rules has come from consumer campaign groups, who wanted to see a UK-style ban on junk food ads to young people.
"Will these health messages be understood by young people?
I doubt it," said Jean-Paul Geai, of France's largest consumer group, UFC Que Choisir .
"Instead of following the opinions of public health experts, or the UK and Sweden, which have banned television adverts aimed at children, France has succumbed to the industry sirens."
A test message that Que Choisir placed on a breakfast cereal advert last year was missed by almost half the people who saw it, the group claimed.
Debate has raged in several countries over the amount of influence adverts can have on young peoples' purchasing choices.
Industry has played down the link, citing several other factors.
But Que Choisir claims adverts "play an active part in children's awareness of food and diet".