The Made In Chelsea star was on hand to model the a-mazing flower dress created by Joseph Massie, although we reckon Rosie's own outfit was pretty gorgeous too!
The starlet stepped out in a little white dress complete with a high neckline, a thigh-skimming hem and cape sleeves.
But while Rosie's wardrobe must be packed with designer buys from Sloane Street, this time her frock cost a very reasonable £38 from Rare London.
The stylista - who runs her own fashion blog - accessorised with a chunky boyfriend watch and a gold coin necklace, while rocking a twist on your usual French manicure by opting for navy tips instead.
She added a touch of interest to her ensemble with black and gold strappy heels, finishing her look with a voluminous bouncy hairdo and a few dainty rings.
And it seems Rosie had a fab time at the event, tweeting: "It was so much fun meeting everyone at @TouchwoodTweets today."
She later added: "Back in London now need to get home and into bed."
We hear ya sister!
What do you think of Rosie's outfit? Let us know by commenting below as the best comment across the site each week wins £50!
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