The message from the Queen and Prince Philip is especially heartwarming.
Through the Royal Family Instagram account, the Queen and the Duke shared a sweet photo taken by Annie Leibovitz of her Majesty and Princess Anne sitting on a lounge in the Drawing Room of Windsor Castle.
"Wishing The Princess Royal a Happy Birthday!" read the birthday message in the caption, which featured balloon and confetti emojis.
"Born on this day in 1950, Her Royal Highness is the second child of Her Majesty and The Duke of Edinburgh. #HappyBirthdayHRH"
Royal fans also noted that the mother and daughter appeared to be very relaxed in the photo, despite posing for the camera.
One wrote: "One of the best natural royal photos I’ve ever seen."
"This has got to be one of my favourite photos of the Queen and Princess Anne," said another.
"I love this picture. The most personal picture I have ever seen of the Queen," wrote a third.
"One of the beautiful and natural royal family photo I’ve seen! Happy birthday Princess Anne."
According to Express, the Drawing Room in Buckingham Palace appears to be the preferred room for important photos of the Royal Family between 2018 and 2019.
For more royals news, check out this month's New Idea Royals magazine.