The Zimbabwe-born blonde is wearing two custom-made dresses by Alberta Ferretti, but she's sort of bucking tradition.
Turns out Chels is actually wearing the shorter satin aqua number to the actual ceremony and changing into the longer navy one for the afterparty.
Ferretti, an Italian designer, had this to say about dressing Prince Harry's date for the big day...
"Having the fitting in Chelsy's flat in Chelsea was real fun and exciting," the designer told People. "She's such a modern girl, full of life and very charming. I'm sure she will wear at her best the special outfits that I have created exclusively for her and this momentous occasion."
The 25-year-old dated hard-partying Harry for five years before splitting with him in 2009, but the two appear to be so on again.
Could Chelsy be a princess herself in the future?
It certainly looks promising for her since he's taking her to his big brother's nuptials. Perhaps seeing Wills tie the knot will make Harry wanna put a ring on it too?