"The Prince of Wales has wished his wife, Catherine, a happy birthday, as she turns 43.
"George, Charlotte, Louis and I are so proud of you. Happy Birthday, Catherine. We love you. W."
The princess has returned to royal duties after having preventative chemotherapy treatment as a result of being diagnosed with cancer last year.
Kensington Palace posted the personal written message from William alongside a previously unseen picture of Catherine taken by photographer Matt Porteous in Windsor last summer.
The black and white picture showed the princess looking relaxed and smiling, wearing jeans, a white shirt and a scarf.
It follows in the couple's style of informal pictures, without any traditional royal imagery.
The Royal Family also wished her a happy birthday.
The post also had a photo of Catherine collecting flowers from wellwishers while attending church in Sandringham on Christmas Day.
That was the last time the Prince and Princess of Wales were seen together in public.
It is understood that Catherine is celebrating her birthday at home in Windsor, with William and their three children, Prince George, Princess Charlotte and Prince Louis.
William has previously described 2024 as probably "the hardest year" of his life, with his father, the King, also diagnosed with cancer.
The emotional video, with its emphasis on family and supporting each other in difficult times, was filmed in Norfolk in August.