Catherine, wife of Prince William and mother to the third, fourth and fifth in line to the throne, turned 38 on Thursday.
The occasion was marked by several royal messages on social media.
Clarence House, the official residence of Prince Charles and the Duchess of Cornwall, tweeted four photographs featuring Catherine — including one with Camilla and two with Charles.
The message contained a balloon emoji and wished the royal “a very Happy Birthday.”
Congratulations were also sent by the royal family’s official Twitter account, accompanied by three images of Catherine with the Queen and a link to a page about the work the duchess does.
“Wishing The Duchess of Cambridge a very Happy Birthday!,” it read.
Thanking the public for their “lovely messages,” Kensington Palace released a previously unseen image of the duchess, who is smiling and sitting against a sunny backdrop.
Dressed in jeans, a blue blouse and a brown sweater and sat on a wooden fence, the duchess is photographed by Matt Porteus.
Meanwhile on Instagram, the duchess also received a birthday greeting from her brother-in-law and his American wife.
Followed by emojis of a cake and a heart, the post from the sussexroyal account read: “Wish a very happy birthday to The Duchess of Cambridge today!”
Catherine has three children with Prince William — six-year-old Prince George, Princess Charlotte, four, and Prince Louis, one.
There have been reports over the last year of a rift between the two sisters-in-law, which was later followed by the couples dividing their royal household and charitable foundation. During their royal tour of Africa last year, Prince Harry told a TV documentary that he had “good days” and “bad days” in his relationship with his brother. The Sussexes announced on Wednesday they will leave their “senior” roles in the British royal family, aiming “to become financially independent.”