"The family are delighted and August is very much looking forward to being a big brother," the statement ended.
Princess Eugenie's mum, Sarah Ferguson, was quick to comment on the Instagram post, penning: "Granny heaven ❤️."
She also took to her own Instagram to delight in the news, posting a photo of August splashing around in the rain. The caption read: "You will be sharing puddles Augie! Superb news, Granny heaven... So deeply grateful."
Meanwhile, Eugenie's brother-in-law, Edoardo Mapelli-Mozzi - who is married to her older sister Princess Beatrice - also shared his love, writing: "❤️❤️."
Princess Eugenie and Jack already share a son, August, who will be turning two years old on February 9.
Their first son was safely delivered in London's Portland Hospital, with Buckingham Palace confirming the news.
"Her Royal Highness Princess Eugenie was safely delivered of a son today, 9th February 2021, at 0855hrs at The Portland Hospital. Jack Brooksbank was present," read a statement.
He weighed 8lbs 1oz at the time of birth.
Eugenie also marked the arrival of her first son on her Instagram, sharing a black-and-white photo of her and Jack's hands grabbing his new-born fingers.
"💙💙💙!!" was the caption, which seemingly confirmed the bub was a boy.
Now, August will have a younger sibling. Congratulations to the happy couple!
This article originally appeared on our sister site, Now To Love.