Middleton, 31, and Prince William, 31 welcomed their first child, a baby boy, who is now the third in line to the British throne yesterday at St Mary's Hospital in London.
Celebrities including Bethenny Frankel, Ellen Degeneres and Teresa Giudice offered their well wishes to the new parents.
Former 'Real Housewives of New York City' star Bethenny Frankel posted, "Congratulations to the Royal Family on their new baby boy!"
"It's a boy! So happy for my cousin Kate and the future King of England!," tweeted comedienne Ellen Degeneres.
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Canadian-American actor William Shatner posted, "So pleased to hear that William and Kate had a baby boy. Long live the future king. MBB"
"It's a BOY, Congratulations to Prince William and Kate Middleton, This is SO GOOD (sic)," singer Sophia Grace.
Some celebrities also remembered Princess Diana on the happy occasion.
"I like to think that the beautiful soul of my recently deceased friend was just reborn as the Royal baby!," said 'The Last Song' producer Adam Shankman.