Louis Vuitton Moet Hennessy and Kering made the move amid criticism the industry encourages eating disorders.
Models will have to be at least a French size 34 for women and a French size 44 for men.
The firms will also not use models under the age of 16 to display clothes designed for adults.
“We have the responsibility of building new standards for fashion and we hope to be followed by other players in our sector,” said Antoine Arnault, an LVMH director.
The two French companies’ brands also include Givenchy, Yves Saint Laurent, Alexander McQueen and Stella McCartney.
France passed a law in 2015 banning ultra-thin models from working in the country’s fashion industry.
Companies could be fined up to 75,000 euros (£69,000) or its bosses jailed for up to six months if they breach the law.
Kering and LVMH said their charter will go further, adding that models will be given a psychologist or therapist while at work.
Models must also be able to present a valid medical certificate that they are fit to work.
The charter will come into effect before Paris Fashion Week this month.