Royal family and fans were delighted with the announcement.
"The most perfect angel in the world," wrote Kitty's sister, Amelia Spencer.
"I didn't know you became a Mom!! Congratulations❤❤❤❤❤❤," one follower commented.
"What wonderful news dear Kitty! Congratulations on your precious little one🤍🤍🤍," another penned.
The delightful news comes almost three years after Kitty tied the knot with her fashion tycoon husband Michael Lewis in Rome.
Kitty and Michael wed after three years of dating. For the ceremony, the now-33-year-old wore five custom dresses from Dolce and Gabbana.
Two of her gowns bore a resemblance to her Aunty Diana, as well as Princess Catherine's wedding dress.
Kitty and Michael originally announced their engagement in 2019, but had to postpone their wedding due to the COVID pandemic.
While some royals, like Princess Beatrice, opted for reduced wedding ceremonies during lockdown, Princess Diana's niece and her then-fiancé chose to wait.
They hosted A-list guests – including the likes of singer Pixie Lott and actor Idris Elba – at the five-star Hotel de la Ville in Rome.