The supermodel siblings colour coordinated - albeit accidentally, according to Lottie - both opting for show-stopping red gowns on Monday 16 May.
Kate wore a one shouldered dress from the Halston archive, while Lottie chose a floor-length, backless style by Dior Couture.
Both Moss sisters were snapped on Instagram, sharing their backstage looks.
Kate was filmed by makeup artist and cosmetics mogul Charlotte Tilbury, who recreated her signature Dolce Vita look on the supermodel.
Lottie also took to the social media app to share her outfit, along with revealing that their matching red outfits were "#notevenplanned".
See more stunning red carpet looks from the 69th Cannes Film Festival in our gallery below: