The star gave birth to the little one yesterday afternoon, her third child with long-term boyfriend Scott Disick.
The couple already have two children together, two-year-old Penelope and five-year-old Mason.
To make the new arrival even more special, yesterday also happened to be little Mason's fifth birthday - so the family will have a double celebration every year from now on.
Although Kourtney did not officially announce the birth on social media, she did share some sweet updates throughout the day, including this picture of one of Penelope's dolls in the new baby's cot, with the caption, "Nap time! Someone's already using our new crib."
Auntie Kim made sure Mason wasn't feeling left out on his big day by organising a special family trip to Disney On Ice at the Staples Centre in Los Angeles. Penelope and Mason were joined by Kim's one-year-old tot North West, and Zeplin Black, the adorable son of make-up artist Joyce Bonnelli.
Kim shared this cute snap on Instagram of the foursome:
Congratulations to the growing Kardashian family!
