The patriotic outfit was fitting for St George's Day, and Kate, 36, looked as flawless as ever as she paired the shift dress with nude heels.
It was also the spitting image of the frock Prince William's late mother Diana wore following the birth of Prince Harry, 33, in September 1984.
It's not the first time Kate has taken style inspiration from Diana, the 20th anniversary of whose death was marked by William and Harry last year.
When she welcomed her first son, Prince George, in 2013 she wore a blue spotted Jenny Packham dress which was the spitting image of the gown Diana wore following William's birth.
Mum-of-three Kate went into labour first thing in this morning and the new prince was born at 11.01am weighing 8lb 7oz.
For the third time running, Kate chose to greet the adoring crowds in a bespoke dress by Jenny Packham.
She paired the gown with a classic pair of classic court shoes by one of Holly Willoughby's favourite designers, Gianvito Rossi.
The shoes cost £495, and are available here.
While Kate looked immaculate with her signature blow dry, thanks to hairstylist Amanda Cook Turner.
She wore the Queen's pearl drop earrings to finish the look, and wrapped the baby prince in a white shawl by GH Hurt & Son, a Nottingham-based brand who also made blankets for George and Charlotte as newborns.
Mum in red like Princess Diana
The Duchess of Cambridge wore a loose-fitting, knee-length dress with white Peter Pan collar, a patriotic look for St George’s Day, as she held her son.
It was a bespoke number from Jenny Packham, who has designed each of Kate’s post-birth dresses, and looked perfect alongside Wills in head-to-toe blue.
It was seen as a tribute to Princess Diana who wore a similar outfit when she first stepped out with baby Harry in 1984.
Diana chose a long scarlet jacket by Jan van Velden, with white shirt and red bow.
Kate completed her polished look with classic nude court shoes, £513, by Gianvito Rossi.
The new mum had help from her stylist and PA Natasha Archer, spotted leaving St Mary’s Hospital in London an hour after the birth.
Hairdresser Amanda Cook Tucker was also believed to be on hand.
When Princess Charlotte was born in 2015, Kate opted for a buttercup print silk shift dress, also by Jenny Packham, for the event.
On the same spot 22 months earlier, the royal mum had also selected an outfit by the same designer – that time a pale blue dress with white polka dots.
The simple shift dress had a high neck and capped sleeves, and was paired with a pair of mid-height beige heels.
The Royal family are said to be delighted with the news, with the Queen informed of the fifth-in-line to the throne's arrival earlier today.
In a statement, Kensington Palace said: "Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs.
"The baby weighs 8lbs 7oz.
"The Duke of Cambridge was present for the birth. Her Royal Highness and her child are both doing well."
READ MORE: Our coverage of the baby boy's arrival
- Holly Willoughby today used a 'psychic banana' to correctly predict Kate Middleton would give birth to a baby boy
- Another couple who welcomed a newborn today found themselves snapped by the world's media on the steps of the Lindo Wing
- Now that we know the tot is a boy – what will his name be?
- Princess Charlotte has made history as the first royal female to retain her claim to the throne
- The new baby was born at 11.01am – find out all the details from the birth
- Eager fans have camped outside the hospital for days in the hopes they will catch a glimpse of the baby
- We took a look inside the Lindo Wing, where Kate has given birth to all three of her children
The Queen today celebrated the birth of her latest great-grandchild with a horse back ride at Windsor Castle.
It's the first time in history that a boy will not precede his older sister in the line of succession, with Princess Charlotte remaining fourth, behind brother George.
Last month we told how Prince Charles fuelled rumours that his wife Camilla Barker Bowles might become queen, after a statement saying she will become "Princess Consort" was removed from his website.
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