The King shared an adorable baby photograph to mark the Prince of Wales's 42nd birthday on Friday.
The black and white portrait taken in 1982 shows the little Prince smiling and sitting on his father's lap as then Prince Charles beams affectionately at his son.
"Happy Birthday to The Prince of Wales!" the caption read.
Meanwhile, the Princess of Wales declared "We all love you" in a personal tribute to her husband as she released a fun photograph of William and their children jumping in the air on a Norfolk beach.
It comes just days after Prince William posted a childhood snap with Charles to mark Father's Day last Sunday.
"Happy Father's Day, Pa," the social media post, personally signed by William, said.
Kensington Palace also released a picture of William with his three children on a beach in Norfolk, taken by the Princess of Wales.
Earlier this week, the Prince of Wales shared a carriage with his stepmother, Queen Camilla, at Royal Ascot, where he was joined by his in-laws, Carole and Michael Middleton, his cousins and close friends.
On the eve of his birthday, William flew to Germany to watch England draw 1-1 against Denmark. The Prince was reunited with King Frederik of Denmark and his daughter, Princess Josephine, with the royals posing for a photo ahead of the match.
All about Prince William
Prince William Arthur Philip Louis of Wales was born second in line to the throne at St Mary’s Hospital, Paddington, at 9.03pm on 21 June 1982.
He weighed 7lb 1 1/2oz and was the first-born son of the then-Prince and the Princess of Wales.
At just nine months old, he accompanied his parents on a six-week official visit to Australia and New Zealand, which was a break from tradition with royal babies usually left at home in the care of nannies.
His mother, Diana, Princess of Wales tragically died in a car crash in Paris in 1997 when William was 15 and his brother Harry was 12, with the young princes walking behind her coffin during her funeral procession.
William met Kate Middleton at St Andrews University in Fife, and the pair married at Westminster Abbey in 2011 after dating for more than eight years.
They welcomed their first child Prince George in 2013, followed by Princess Charlotte in 2015, and then Prince Louis in 2018.