The 30-year-old looked splendid wearing a vibrant Alexander McQueen frock with long sleeves and a ladylike pleated skirt, not unlike the Emilia Wickstead number she repeated last week.
Kate topped off the look with her favourite nude LK Bennett pumps, a red clutch and matching Sylvia Fletcher hat from James Lock & Co enlivened by delightful floral details - it rather reminds us of the maple leaf headgear she chose for Canada Day last year, also designed by Fletcher.
And note that maritime-themed brooch featuring two silver dolphins, which is a gift from the Royal Navy Submarine Service, of which Prince Wills is Commodore in Chief.
The all-red ensemble ensured Kate stood out from the crowd and was perfectly co-ordinated with the royal barge, which featured red thrones seats, 10,000 freshly-cut flowers, tremendous gold busts and a LOT of flags.
Prince William looked mighty dapper by her side wearing his military uniform decorated with medals. As the weather turned gloomy, the Duchess cracked out a Strathearn tartan scarf to wrap around her neck but her gloriously bouncy hair, in a half-up and half-down style, stayed miraculously frizz-free.
The dress comes from McQueen's Pre-Fall 2011 collection and has previously been seen on Tulisa Contostavlos and Kim Kardashian (would you Jubilee-ve it?!).
The X Factor sweetheart first wore the frock, costing £1,195, at the Liverpool auditions in July last year with black Nicholas Kirkwood heels and a fishtail plait-do.
Then Kim followed suit one month later while out and about in NYC, accessorising the look with snake print Christian Louboutins, a black purse and statement necklace.
But Sarah Burton has customised it for the DoC with long sleeves and a suitably modest knee-skimming hemline.
As Grazia's style director Paula Reed noted during her appearance on the BBC, 'Kate has taken a leaf out of the Queen's style book by realising that a bit of slim-line tailoring keeps you looking sharp and cool in a situation like this'.
