First Minister Peter Robinson has led congratulations to the Duke and Duchess of Cambridge following the birth of their son this afternoon.
He extended his "very best wishes" to the couple "on behalf of everyone in Northern Ireland".
"I was honoured to attend the celebration of their wedding in April 2011 and I wish them every health and happiness in the next chapter of their lives together," he said.
DUP MP Jeffrey Donaldson took to Twitter to say: "Congratulations to the Duke & Duchess of Cambridge on the birth of their son. A Prince and future King of our United Kingdom is born."
Ulster Unionist leader Mike Nesbitt also welcomed the birth.
"This is brilliant news for William & Kate, and all the Royal Family. First and foremost it is great to hear that mother and child are both well, and we wish the new parents all the very best.
"The United Kingdom can now enjoy the next few days as we celebrate the birth of the latest addition to the Royal Family."
SDLP MLA Conall McDevitt tweeted: “A birth is wonderful. Many thousands born today will some day share their birthday with a King. Congrats to all the mums and dads, royal and not.”
Alliance MP Naomi Long said: "Congratulations to Duke and Duchess of Cambridge on the birth of their baby boy. Glad to hear both mother & baby are doing well".
Ulster Unionist MLA Jo-Anne Dobson tweeted that her thoughts were with William and Kate, adding that the “whole Royal family will be beaming tonight - a Royal baby boy to captivate and melt the hearts of a nation.”
Meanwhile, the Prime Minister said the "whole country will celebrate" the birth of the royal baby as news of the child's arrival spread across the globe.
David Cameron offered his congratulations on Twitter where the worlds of showbiz, politics and sport appeared to share in the Duke and Duchess's delight.
He wrote: "I'm delighted for the Duke and Duchess now their son has been born.
"The whole country will celebrate. They'll make wonderful parents."
Meanwhile Oliver Hicks, 31, a close friend of Prince William, said he was thrilled to hear the "fantastic" announcement.
The adventurer said he had been hoping for news since completing the first recorded kayak crossing of the North Sea between Britain and Norway, on Friday.
"It's just such fantastic news," he said.
"Presumably it's also much easier that it's a boy. I hope I'll get to see him soon. I can't wait to see all of them."
Archbishop of Canterbury the Most Rev Justin Welby was one of the first to speak of his joy on Twitter.
"Delighted for the Duke and Duchess of Cambridge as they welcome their baby boy. May God bless them all with love, health and happiness," he said.
Labour Leader Ed Miliband added: "Many congratulations to the Duke and Duchess of Cambridge. I wish them and their son all happiness and good health."
Even former Spice Girl Emma Bunton expressed her excitement - echoing a sentiment felt among the hordes gathered outside Buckingham Palace.
"It's a boy!!! Special times ahead for Kate and William," she said.
Deputy Prime Minister Nick Clegg said the "good news" would make the "whole country smile" as he offered his congratulations on the micro-blogging site.
And First Minister of Wales Carwyn Jones wished the young royals the "very best" as they became parents for the first time.
"On behalf of the people of Wales, I would like to congratulate the Duke and Duchess of Cambridge on the birth of their baby boy," he said.
"The couple already have established and strong links with Wales, choosing Anglesey to start their married life, and they will always have a very warm welcome home here as a family.
"I would like to wish the Duke and Duchess all the very best as they enter their journey into parenthood."
Scottish First Minister Alex Salmond added: "I am sure that people across Scotland will be absolutely thrilled to hear the news of the birth of a baby boy to the royal couple and will want to join me in wishing the proud parents many congratulations."
Mayor of London Boris Johnson said: "Congratulations to the Duke and Duchess of Cambridge on their new arrival."
Foreign Secretary William Hague added: "My warmest congratulations to the Duke and Duchess of Cambridge.
"Wonderful news and a happy moment for our country and Overseas Territories."
Canada's prime minister Stephen Harper sent his "heartfelt congratulations" to the couple and the royal family on behalf of all Canadians.
Outgoing Chief Rabbi Lord Sacks tweeted "Mazal Tov" to the happy couple.
"We wish them blessings, good health, and joy in becoming parents," he added.
Tessy Ojo, CEO of the Diana Award scheme, an initiative which recognises teenagers who make outstanding contributions to their communities, spoke of Prince William's mother.
"Today Diana, Princess of Wales, would have been a proud grandmother," she said.
"Her legacy continues through the inspirational work of these young people who carry this honour, set up in her memory, with pride."