The Duchess of Cambridge gave birth to a daughter on Saturday, adding a new member to the British royal family.
The baby - who is fourth in line to the throne - was "safely delivered", the Kensington Palace announced in a statement.
The Duke of Cambridge was present during the birth of the baby, it said.
Both Catherine and her daughter, who was delivered at 8.34 a.m., are "doing well", the statement added.
"The baby weighs 8lbs 3oz."
The duchess had earlier been admitted to the Lindo Wing at St.Mary's Hospital where Prince George was born in July 2013.
Catherine travelled from the palace with her husband Prince William.
The statement from Kensington Palace 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 latest royal joins the line of succession behind her grandfather Prince Charles, father Prince William and brother Prince George.
The Prince of Wales, who had earlier said he was hoping for a grand-daughter, and the Duchess of Cornwall said they were "absolutely delighted", according to BBC.
Political leaders tweeted their congratulations for the royal family.
"Congratulations to the Duke and Duchess of Cambridge on the birth of their baby girl. I'm absolutely delighted for them," Prime Minister David Cameron wrote on Twitter.
Deputy Prime Minister Nick Clegg said he and his wife Miriam wished "them all the best as their family grows", Xinhua news agency reported.
Labour leader Ed Miliband tweeted: "Congratulations to the Duke and Duchess on the birth of their daughter. Wishing them lots of joy and happiness -- and hopefully some sleep!"
"It's wonderful news that the Duke and Duchess of Cambridge have a healthy new baby girl," the Archbishop of Canterbury Justin Welby said.
"Like many here and abroad, I'm delighted for the royal couple on this special day. May God bless them and both of their children with love, health and joy."
Meanwhile thousands of well wishers gathered outside St Mary's Hospital to catch a first glimpse of the new princess, BBC reported.
There has been lots of speculation about a name from small girls in the crowd, from Disney-inspired Belle and Ariel to Alex, Stella and Rose, the broadcaster said
.
People also wanted selfies with Tony Appleton, who runs a care home in Chelmsford, Essex, but has been a town crier for 30 years and stood on the steps of the Lindo wing to announce the news.
