John Harrison, who is Kate Middleton's first cousin twice removed, said he was "over the moon" that he is now related to the future King of England.
He and his wife Val still live in Hetton-le-Hole, County Durham, where the Duchess of Cambridge's great-grandparents once lived.
Gun salutes are sounding across London to mark the birth of the royal baby.
The King's Troop Royal Horse Artillery and the Honourable Artillery Company are both carrying out the ceremonial salutes in honour of the new addition to the Royal Family.
Gun salutes are fired for the birth of every prince or princess, no matter where their place is within the line of succession, the Ministry of Defence said.
The King's Troop Royal Horse Artillery, wearing full dress uniform, rode past Buckingham Palace to Green Park where they are staging a 41 royal gun salute.
The Honourable Artillery Company (HAC), the City of London's Army Reserve Regiment, are firing a 62 royal gun salute from Gun Wharf at the Tower of London.
Gun salutes will shortly sound across London to mark the birth of the royal baby.
The King's Troop Royal Horse Artillery and the Honourable Artillery Company will both carry out the ceremonial salutes in honour of the new addition to the Royal Family.
A knitted version of the royal baby boy has been spotted outside the Outer West Library in Newcastle.
Deputy Prime Minister Nick Clegg has offered his congratulations to the Duke and Duchess of Cambridge after the birth of their son yesterday.
He said: "This is wonderful news. Miriam and I were absolutely delighted when we heard news and we want to send our heart-felt congratulations".
"It's news that will make the country smile", he added.
Guardsman at Buckingham Palace have played Congratulations during The Changing of the Guard today, to mark the birth of the Duke and Duchess of Cambridge's son.
The Changing of the Guard is underway at Buckingham Palace and today's music will include Congratulations, Royal Salute and The Duke of Cambridge March to celebrate the birth of the royal baby.
Coca-Cola have posted a picture on Twitter in celebration of the birth of the Duke and Duchess of Cambridge's baby boy.
The tweet said: "Congratulations Kate & Wills! A toast to your new arrival..."
The arrival of the royal baby will be marked in tonight's episode of EastEnders after show bosses ordered some last-minute filming to reflect the birth.
A scene showing Dot Branning (June Brown) and Abi Branning (Lorna Fitzgerald) discussing the happy news was filmed last night and will feature in the BBC soap tonight.
A spokeswoman for the show said: "This is such a momentous occasion for the Royal family, the country and of course the residents of Walford that we felt it should be marked in EastEnders history."
Residents of Bucklebury, the village where the Duchess of Cambridge grew up, are celebrating the birth of the Prince of Cambridge.