"Thank you so much for all of your warm wishes," the mother-of-two wrote, alongside a photo of Hart's nursey.
"Evan, Flynn and I have been in baby bliss for the last 17days with our new arrival, baby Hart. We feel so very blessed."
Sadly, Kerr didn't include her second son in the photo, something that probably won't happen any time in the near future, given her 7-year-old, Flynn, rarely features in any of her public posts.
"Words cannot explain how happy we are to welcome our beautiful son into our family. Miranda is doing well and Flynn is excited to be a big brother," reps for Kerr and Spiegel shared with People when Hart was first born, revealing his name is a tribute to Spiegel's grandfather.
"Thank you for the kind words and wishes during this very special time."