In one of the delightful snaps, Kate and Camilla are seen dressed in their Sunday best as they arrive for the annual Trooping The Colour parade in 2017.
A follow-up photo shows Kate enjoying a private joke with hubby Wills and her father-in-law, Prince Charles, who is seen laughing out loud.
Another candid snap shows the Duchess sitting in a royal carriage, next to Prince Charles, who dressed in official regal attire, including a cap with an oversized feather.
Enthusiastic royal fans were quick to take to Instagram to comment on Camilla’s sweet tribute, with one person writing: “Happy Birthday to The Duchess of Cambridge.”
Another person stated: “Wishing a very happy 38th Birthday to The Duchess of Cambridge.”
A third person added: “Lovely photos of the Duchess with her parents in laws and her husband. Well done for been a wonderful and modern example for women around the world.”
Kate’s birthday tribute comes after Kensington Palace released a stunning new portrait of the Duchess to mark her 38th birthday on Thursday.
The photo, which shows Kate looking relaxed in a grey wooly jumper and blue jeans, is believed to have been taken at her Norfolk home, Anmer Hall, in September 2018.
"Thank you everyone for all your lovely messages on The Duchess of Cambridge’s birthday," the caption on the picture read.
The Duke and Duchess of Sussex replied: "Wishing a very happy birthday to The Duchess of Cambridge today!"