She posed for the cameras in a black and silver mesh-effect Chanel catsuit before the show outside the Grand Palais in the French capital.
She is the luxury brand’s youngest “trendsetter”, following Hollywood star Johnny Depp’s 16-year-old daughter Lily-Rose who was named a Chanel ambassador in July.
Chanel’s designer Karl Lagerfeld said last year that he had taken Violette d’Urso, the 15-year-old daughter of his former inspiration Ines de la Fressange, as his latest “muse”.
After Tuesday’s catwalk show the 82-year-old fashion legend reappeared to take his bow holding the hand of his seven-year-old godson Hudson Kroenig — son of the model Brad Kroenig — whose coat echoed the collection.
Chanel said Willow Smith had been wearing their clothes for “public appearances since 2013 and her first encounter with Karl Lagerfeld was in 2014 when he photographed her for the American magazine V.”
Smith recorded her first album at the age of 10 and supported singer Justin Bieber the following year on a part of his European tour.
Her rapper and actor brother Jaden Smith appeared in a skirt earlier this year in a photoshoot to promote the Louis Vuitton spring-summer collection.
Paris fashion week has been more diverse than ever in terms of age and race of those on the catwalk, despite a row over rapper Kanye West’s favourite designer Demna Gvasalia only casting white models for his two shows.