He didn't write a caption to go along with the picture, but his followers understood the significance of it.
"Thinking of you today Earl Spencer , Diana will truly never be forgotten .❤️," one woman commented, while another declared, "Our beautiful Princess will always be remembered."
Earlier this summer, Charles revealed that he visits his sister's grave a few times each year.
"Every landmark day, such as birthday, Mother's Day, I always take flowers," he shared on Good Morning Britain. "I do go a lot, and it's an oasis of calm, and it's a lovely place to go."
Charles' most recent royal outing came on July 1, when Prince Harry and Prince William unveiled a statue of their mother on what would have been her 60th birthday.
Due to COVID-19 guidelines, only a select few were invited to the ceremony at Kensington Palace's Sunken Garden, and Charles — as well as their two sisters Lady Sarah McCorquodale and Jane Fellowes — made the cut.
During the special day, the brothers were seen shaking hands with their uncle, and the men all shared a few laughs together.
Since Harry and Meghan Markle moved to America, they haven't been able to spend much time with Charles, but the latter is excited to meet their new daughter, Lilibet Diana, who was born on June 4.
"I am of an age where I now have 10 great-nephews and great-nieces, but everyone genuinely is a joy," he gushed. "So I genuinely can say yes, I very much look forward to that."