The health ministry says the aim is to fight eating disorders and inaccessible ideals of beauty.
Digitally altered photos will also have to be labelled from 1 October.
Images where a model's appearance has been manipulated will need to be marked photographie retouchée (English: retouched photograph).
A previous version of the bill had suggested a minimum BMI for models, prompting protests from modelling agencies in France.
Models must now provide a doctor's note when applying for jobs
But the final version, backed by MPs 2015, allows doctors to decide whether a model is too thin by taking into account their weight, age, and body shape.
Employers breaking the law could face fines of up to 75,000 euros (£63,500; $82,000) and up to six months in jail.
"Exposing young people to normative and unrealistic images of bodies leads to a sense of self-depreciation and poor self-esteem that can impact health-related behaviour," said France's Minister of Social Affairs and Health, Marisol Touraine, in a statement on Friday, French media report.
France is not the first country to legislate on underweight models - Italy, Spain and Israel have all done so.
Anorexia affects between 30,000 to 40,000 people in France, 90% of whom are women.
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