The couple now have three children, and their new addition shares a birthday with their son Mason, 5.
Kourtney, 35, posted a message saying she is "blessed and honored" to be Mason's mommy today.
Kourtney and Scott also have a daughter Penelope, 2.
She said, "I am so happy we're having a boy," Kourtney says. "It's going to be perfect for our family."
Congrats Kourtney and Scott!