Kate Middleton is showing love for her husband, Prince William, on his 42nd birthday. 

On Friday, the Princess of Wales — who is currently undergoing treatment for cancer — took to social media to pay tribute to William by sharing a fun family portrait. 

"Happy birthday Papa, we all love you so much! Cx," the Instagram caption read, alongside a photo of William and their three children — Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6 — jumping in the air while holding hands. 

PRINCE WILLIAM'S DEVOTION TO KATE MIDDLETON DEEPER THAN EVER AS HEIR CELEBRATES 'BITTERSWEET' BIRTHDAY: EXPERT

Fans, including some famous ones, were quick to jump in and offer their own birthday wishes.

‘Ted Lasso’ star Hannah Waddingham wrote, "Epic pic! Happy Birthday Sir!"

"Happy birthday ! Fatherhood: example example example and you do it beautifully," Bear Grylls added. 

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"One of the best family portraits ever. Captures all the joy, love and laughter. Utterly brilliant," another fan wrote. 

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King Charles also shared a sweet birthday message for his eldest son on the royal family's Instagram Stories. 

"Wishing The Prince of Wales a very happy birthday," the post read, alongside a black-and-white throwback photo of a young Charles and William as a baby. 

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Since the Princess of Wales announced her cancer diagnosis earlier this year, Prince William has stepped up as he continues his royal duties. 

"William could not have been a more devoted husband than he has been to Kate," Christopher Andersen, author of "The King," recently told Fox News Digital. "That’s ever more evident in the way that he has stepped up to reassure the children and, at the same time, take up some of his father’s duties while Kate is undergoing chemotherapy."

"Kate’s public statements about her cancer battle have been nothing less than stunning in their graciousness and insight," Andersen shared. "William recognizes that."

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As the patriarch of their family, Andersen said that William fully understands the children look to him for reassurance during this tough time. 

"William also realizes that he is the only one who has to reassure the children that this is just a rough patch and their mother will get through it — not to mention their grandfather, who the children know is also suffering from cancer," Andersen said, referencing King Charles' cancer battle.

"George, Charlotte and Louis are also very close to their grandfather the king, and people often forget that," Andersen continued. "It’s up to William to make sure that not only are they reassured about their mother’s situation, but also their beloved grandfather’s. So much is on William’s shoulders as he turns 42. It really is rather staggering."

Fox News Digital's Stephanie Nolasco and Ashley Papa contributed to this post.