Kate went into labour in the early hours of the morning and was taken to the St. Mary's hospital in Paddington where she had her baby. The baby weighed 8lbs 3oz (3.7kg) at birth.
Prince George on his way to see his baby sister with his Dad, stopped to give reporters a cute wave.
Several congratulatory messages have been pouring in from celebrities and world leaders with #Royalbaby trending on Twitter.
And in honour of the royal baby, the Royal Navy tweeted a picture of sailors also known as the "Queen's Frigate," in a formation spelling out the word "sister." So sweet!
Congratulations to the couple!