While we were guessing that she might don something like a vintage Christian Dior or a Vivienne Westwood, the designer herself, and Kate's close friend, says otherwise.
'No, I think she's designing it herself. She has done her own fashion range and she knows about clothes. She knows what she is doing, she doesn't need my help..."
The supermodel is reportedly set to marry her rocker beau Jamie Hince on 2 July this year, just over two months after the royal wedding.
What kind of dress do you think Kate will wear? Vintage and lace? Short and chic? Let us know below...