The picture showed William and Prince George, Princess Charlotte and Prince Louis standing with their backs turned to the camera, looking out at the beach.
“We love you, Papa,” the caption read. “Happy Father’s Day 💕 G, C & L.”
The photo was taken by the Princess of Wales earlier this year.
In addition to the photo of William and his children, the future king also shared a photo for his dad, King Charles.
“Happy Father’s Day, Pa” the caption said, which William signed “W.”
Kate revealed on Friday that she was attending the celebrations this weekend in a deeply personal message about her chemotherapy treatments.
“I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days,” she said in her message. “On those bad days you feel weak, tired and you have to give in to your body resting.”
“But on the good days, when you feel stronger, you want to make the most of feeling well. My treatment is ongoing and will be for a few more months,” Kate went on, adding that she is “taking each day as it comes, listening to my body, and allowing myself to take this much needed time to heal.”