Sharing a photo of her daughter as a child wearing a sweet white dress and sniffing a flower that she'd picked, Sarah called Beatrice "one of the most special people on this planet".
She also described the time that the Princess was born, 8.18pm, as "one of the greatest and most proud moments of my life".
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Sarah's full message read: "Happy Happy Birthday My Dearest Beatrice. 8.8.88 at 8.18pm was one of the greatest and most proud moments of my life. You are unique and simply one of the most special people on this planet. Your ever loving and devoted Mum."
Sarah shared the sweet post on Instagram
The Queen also marked Beatrice's birthday on Saturday, dedicating a sweet message to her granddaughter.
Sharing a photo of the pair laughing together following a Maundy Thursday Service, Her Majesty penned on Twitter: "Wishing Princess Beatrice a very Happy Birthday!" followed by a cake and balloon emoji.
The tweet added: "Her Royal Highness is The Queen's 5th grandchild. This photo was taken when The Princess joined her grandmother at a Maundy Thursday Service at York Minster."
While the Princess is likely to spend the day in private with her husband Edoardo Mapellli Mozzi, members of the royal family usually post birthday tributes to family members on their social media accounts.
Beatrice Elizabeth Mary was born on 8 August 1988 at the Portland Hospital in London, but her name was not announced until almost two weeks after her birth. She was baptised in the Chapel Royal at St James's Palace in December 1988.
Her younger sister Princess Eugenie arrived in March 1990.
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