The tot was born on 18 September 2021 at the Chelsea and Westminster Hospital, and no doubt her proud parents will have made the day special for her.
Sienna's birthday comes just days after Beatrice and Edoardo wowed on the red carpet at the Vogue World event in London.
Edoardo looked smart in a white tuxedo, while Beatrice stunned in a caped floral Richard Quinn dress with sparkling earrings and a bold red lip.
The couple were also joined at the star-studded bash by Beatrice's younger sister, Princess Eugenie, who wore a green satin Fendi dress in her first public appearance since welcoming second son Ernest in May.
Beatrice and Edoardo are yet to share an official photograph of their daughter Sienna, opting to post an image of their newborn's footprints after her birth in 2021.
Earlier this year, Eugenie shared a rare snap of her niece with her eldest son, August, as they enjoyed a trip to London Zoo. The sweet photograph showed the royal cousins peering into the penguin tank.
And back in April, when a new portrait of the late Queen Elizabeth II with her grandchildren and great-grandchildren was released, there appeared to be a photograph of Edoardo and Sienna at her christening on the table in the background.
Proud grandmother Sarah, Duchess of York has also shared a number of anecdotes about her three grandchildren, including Sienna's nickname. Find out in the clip below…
In an episode of her podcast Tea Talks with The Duchess and Sarah, she also spoke about celebrating Beatrice's 35th birthday in August.
Author Sarah said: "I hid many different fairies in the garden, and I sent Sienna on a fairytale hunt to find the fairies, with wings on. She lasted with the wings on for quite a while before Beatrice and I put the wings on. And then we had a fairy picnic, once she'd found the fairies and that was a magical day for Beatrice's birthday."
Beatrice and Edoardo marked their third wedding anniversary in July, and the property developer regularly gushes about his wife on his Instagram account.
Sharing a snap of the pair from the red carpet, Edoardo wrote: "My beautiful wife, Bringing joy to my life."
Edoardo also has a seven-year-old son, Christopher Woolf, known as Wolfie, from his previous relationship with architect Dara Huang.
Listen to HELLO!'s A Right Royal Podcast and find out the surprising shoes the late Queen wore under her robes...