"Happy Birthday Baby Girl. You bring us so much love and joy every day," he wrote in the caption. The sweet image showed little Sienna, with her back to the camera, wearing a long-sleeved pink and red floral dress with a wide-brimmed straw party hat.
In response, Edoardo's ex penned: "Happy birthday Cece [heart emojis]."
The American-born architect was previously engaged to Edoardo, with the former couple sharing son Wolfie, who was born in 2016.
Although Wolfie has been seen at the late Queen's Jubilee celebrations and for the Christmas Day service at Sandringham in December, Beatrice and Edoardo have not shared any official images of Sienna since her birth in 2021, preferring to keep their daughter off their social media accounts.
The young tot has also not joined her parents for any formal public events. Sienna is currently tenth in line to the British throne, but is not likely to become a full-time working royal in the future. Meanwhile, earlier this year, Dara confessed what Wolfie thinks of his little sister in a candid Instagram video.
In the clip, the interviewer asked her: "How does Wolfie feel about his little sister?" and to the question she responded: "Wolfie loves his little sister, and they are so cute together," with a big smile on her face.
Wolfie resides with his mother Dara in Kensington, London in a rental property that has been designed to perfection.
He shares a close bond with his stepmum Beatrice. The royal previously handed out an award at the 10th annual Oscar's Book Prize winner's ceremony and during her speech, she spoke about her stepson's love of reading, saying: "The award holds a very special place in my heart.
"My stepson, who is now seven, and I eagerly look forward to receiving the books and going through them together. Growing up in a world surrounded by books, we can take for granted that not everyone is so lucky.
"We need to remember in a world of screens and AI, it is moments of community, including between parents and children that we need to keep alive."