Along with a photo of Bea and Edo looking loved-up near a waterfall, Eugenie penned: "To my dearest Beabea and Edo, Congratulations on your new angel.
"I can't wait to meet her and I'm so proud of you. We're going to have so much fun watching our children grow up. Love Euge."
The 31-year-old also added a few words specifically meant the new bub.
"To my new niece, I love you already and think you're just awesome from the photos.. we're going to have so much fun together. Love your Auntie Euge 🥰🥰💕💕😘."
Along with Eugenie, Bea's uncle Prince Charles and his wife Camilla Parker-Bowles also shared their support for the new parents.
Sharing the official announcement post from the royal family to their story, they added: "Congratulations to Princess Beatrice and Edoardo Mapelli Mozzi on the birth of their daughter 🎉."
WATCH: Sarah Ferguson references herself being a "granny" following the birth of Princess Eugenie's son (Article continues after video)
Last night, it was announced that Bea and Edo had welcomed their first child via an official statement to the royal family website.
"Her Royal Highness Princess Beatrice and Mr Edoardo Mapelli Mozzi are delighted to announce the safe arrival of their daughter on Saturday 18th September 2021, at 23.42, at the Chelsea and Westminster Hospital, London.
"The baby weighs 6 pounds and 2 ounces.
"The new baby’s grandparents and great-grandparents have all been informed and are delighted with the news. The family would like to thank all the staff at the hospital for their wonderful care.
"Her Royal Highness and her child are both doing well, and the couple are looking forward to introducing their daughter to her big brother Christopher Woolf."
Congratulations to the new parents!
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