The two conglomerates said they had signed a charter for "the well-being of models". The charter also forbids the hiring of girls under the age of 16 to pose as adult models for shows or shoots.
In 2015, France passed legislation outlawing ultra-thin models from working in the country's fashion industry. Fashion agencies face fines of up to €75,000 ($89,527) or imprisonment of up to six months if they breach the law.
Kering, which owns Gucci and Saint Laurent, and LVMH, which owns Louis Vuitton and Christian Dior, said their charter will go further, adding that all their fashion brands commit to banning models below size 34 for women and size 44 for men in French measurements from their casting requirements.
Size 32 in France corresponds to XXS or size 6 in Britain, and size 0 in the United States.
Fashion industry insiders have long said that clothes hang and drape better on tall, androgynous women, while western cultures often associate thinness with wealth, youth and desirability.
In 2010, Isabelle Caro, an anorexic 28-year-old former French fashion model, died after posing for a photographic campaign to raise awareness about the illness.
The charter will come into effect ahead of this month's Fashion Week in Paris.
By Dominique Vidalon; editors: Richard Lough and Louise Heavens.