Beatrice, who gave birth on Sept. 18th, announced she and her husband have named their daughter Sienna Elizabeth Mapelli Mozzi.
"We are all doing well and Wolfie is the best big brother to Sienna," Beatrice shared Friday on Twitter, referencing her stepson, Christopher Woolf, the son of her husband, Edoardo Mapelli Mozzi.
Sienna is the 12th great-grandchild for Queen Elizabeth and the late Prince Philip.
The newborn was born at Chelsea and Westminster Hospital in London and weighed 6 pounds and 2 ounces, according to Buckingham Palace.
"The new baby’s grandparents and great-grandparents have all been informed and are delighted with the news," the palace said in a statement announcing Sienna's birth. "The family would like to thank all the staff at the hospital for their wonderful care."
Beatrice, 33, is the eldest daughter of Prince Andrew and Sarah Ferguson.
She and Mapelli Mozzi, wed in July 2020 in a private ceremony that was scaled down due to the coronavirus pandemic.
The couple's daughter is the fourth great-grandchild for Queen Elizabeth born this year.
Beatrice's sister, Eugenie, welcomed a son, August Philip Hawke Brooksbank, in February. The two babies are the first grandchildren for Sarah and Andrew, who continues to face intense scrutiny over his relationship with convicted sex offender Jeffrey Epstein.
Another granddaughter of the queen, Zara Tindall, the daughter of Princess Anne, gave birth to her third child, a son, in March.
Duchess Meghan, the wife of Prince Harry, the youngest son of Prince Charles, welcomed their second child, a daughter, in June.
Harry and Meghan, the Duke and Duchess of Sussex, named their daughter Lilibet, a family nickname for Queen Elizabeth.