The 40-year-old supermodel will front the fashion house's upcoming
Autumn/Winter campaign, taking over from German beauty Claudia Schiffer,
who starred in their most recent series of advertisements.
Christy - who has two children, Grace, five, and three-year-old Finn with husband Edward Burns -fronted YSL's campaign five years ago.
Her latest pictures, which were taken by photographers Inez van Lamsweerde and Vinoodh Matadin, will appear in fashion magazines from later this month.