The former Sonny With a Chance star and her husband Chris Carney welcomed little KJ early on Tuesday (14Aug12).
Carney announced the happy news with a post on his Twitter.com account, which read, "@therealTiffany and (I) just had our first Son KJ!!! 8-14-12 So amazing!!!"
The newborn's arrival should come as no surprise to fans - Thornton alerted followers she was going into labour on Monday night (13Aug12), tweeting, "Epidurals are amazing".
The couple wed in late 2011.