The young royals smiled and waved as they walked hand-in-hand with their dad Prince William into the hospital earlier this afternoon.
Princess Charlotte appeared to be particularly excited to meet her new baby brother, waving at waiting media as she and her big brother Prince George walked next to their dad.
They were later taken back to Kensington Palace ahead of Wills and Kate's departure later today.
The royal couple's new baby boy was born at St Mary’s Hospital, Paddington this morning, weighing in at 8lbs, 7oz, at 11.01am.
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."
They added: "The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news."
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 but in front of her new male sibling.
Many are now hopeful of a glimpse of the new family-of-five, with many hoping to see the latest addition to the royal family.
Excitement had today reached fever pitch when Kate's stylist was seen leaving the Lindo Wing – sparking speculation that Kate had already been prepared to step out in front of the public to introduce her son.
Many royal fans have been camped outside the hospital for weeks, eager to catch a sight of the little boy – Kate and William's second son.
Messages of congratulations to the young family have already started to flow in – with Prime Minister Theresa May tweeting: "My warmest congratulations to the Duke and Duchess of Cambridge on the birth of their baby boy.
"I wish them great happiness for the future."
The town-crier also declared the happy news, wearing the traditional outfit as he shared the announcement outside the Lindo wing.
A large Union flag was also raised over Buckingham Palace, while a State Standard was raised over Windsor Castle to celebrate the birth of the baby boy.
Other messages of support even came from the Royal Navy, proudly spelling out BOY! on board the HMS Albion.
Celebrities have also been posting messages of congratulations.
Liam Payne wrote on Twitter: "Congratulations to her Royal Highness the Duchess of Cambridge Kate and the Duke of Cambridge, Prince William on the birth of their baby boy! #RoyalBaby".
Speaker John Bercow told the Commons: "I am sure the whole House would want to join me in sending Their Royal Highnesses, the Duke and Duchess of Cambridge, our warmest congratulations on the birth of their son."
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
Defence Secretary Gavin Williamson added: "I'd very much like to associate the Government with your comments and warm wishes."
Kensington Palace this morning had created a flurry of excitement when it was announced that Kate had been admitted to hospital before 6am today.
In the tweet, the palace confirmed: "Her Royal Highness The Duchess of Cambridge was admitted to St. Mary’s Hospital, Paddington, London earlier this morning in the early stages of labour.
"The Duchess travelled by car from Kensington Palace to the Lindo Wing at St. Mary’s Hospital with The Duke of Cambridge."
The 36-year-old's two previous births have been fairly quick affairs – Kate had George less than 11 hours after arriving at hospital on July 22, 2013.
Charlotte was born just two hours and 34 minutes after mum Kate as admitted on May 2, 2015.
Kensington Palace had confirmed that the new royal baby was on the way on September 4, 2017 – with bookies quick to take punts at what royal baby number three will be named.
Either way, the new arrival will be given an extremely formal title either way – becoming known as His Royal Highness Prince (Name) of the United Kingdom of Great Britain and Northern Ireland.
Royal fanatics have been pictured celebrating outside the Lindo wing, excited for the first peek at the couple's third baby.
John Loughrey, 63, who is part of a group who have been camped outside the hospital, said: "We are so pleased. We have been here for 15 days.
"I'm so pleased it's St George's Day. St George himself would be very pleased."
Mr Loughrey, from Streatham, south London, said he plans to celebrate the birth with English flags and a portion of fish and chips.
Other royal fans have been sharing celebratory drinks outside the hospital, with one saying: "It's a lovely atmosphere, having been here it's wonderful.
"I wanted Phillipa, so I'm going to have to go with Phillip."
Another added: "I hope it's going to be an Irish name, maybe Patrick.
"It's fantastic to be here and see the new royal baby."
While the Queen has four children, the Duke of Cambridge is the first future king to have a third for more than 120 years.
The Duchess of Cambridge has managed to battle through a number of public engagements this year, despite suffering from another bout of severe morning sickness.
Hyperemesis gravidarum, which causes severe vomiting, can lead to dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.
During her pregnancy, she had to miss Prince George's first day at school as she was too poorly to attend.
But she later improved, making a surprise appearance at Paddington station in October, joining William and Harry at a charity event, where she danced on the platform with Paddington Bear.
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