The frock had a boat neckline and three quarter length sleeves.
It appears to be bespoke, but you can shop a very similar style from the same designer - which is .
The new Duchess of Sussex also donned a hat by legendary milliner Stephen Jones and matching court heels by Carrie Bradshaw's favourite Manolo Blahnick.
Guttingly, , but we'll keep an eye out in case they come back in stock.
The young prince, who was born on St George's Day, April 23, was christened at The Chapel Royal, in St James' Palace, London, by the Archbishop of Canterbury at 4pm today.
Mum-of-three Kate Middleton, looked positively glowing in cream V-neck number by Alexander McQueen and floral Jane Taylor headpiece.
The 36-year-old has previously worn Alexander McQueen for the christenings of her elder children George, four, and Charlotte, three - as well as for Prince Harry and Meghan's recent wedding.
Little Louis' aunt Pippa Middleton also wowed in a bespoke pastel blue outfit by Alessandra Rich, which perfectly complimented her glowing suntan.
The 34-year-old, whose baby is believed to be due in October, hid her baby bump under a suit dress and baby blue hat.
Her cousin Lucy Middleton, 32, who was named as one of Louis' six godparents earlier today, matched Pippa in a blue floral dress, and an uncannily similar hat.
Charlotte also wore a blue frock, while Princes William and Harry donned blue suits and ties - in celebration of the little prince.
We previously revealed how to get Meghan's £1,300 Brandon Maxwell dress for a TENTH of the cost.
While we tried out the bizarre facial exercises behind Meghan Markle's radiant complexion - here's our verdict.
And it appears Meghan is growing out her signature hairstyle - did you spot it?