The former Sonny with a Chance star gave birth to Kenneth James on Tuesday (August 14), E! Online reports.
The baby, already nicknamed KJ, is the first child for Thornton and her husband Chris Carney.
The couple announced their engagement in February 2010 before marrying last November.
The birth was also attended by stars Wilmer Valderrama and Cassandra Scerbo, who have been named KJ's godfather and godmother.
Meanwhile, both Carney and Thornton posted photographs of their new son on Twitter, with Carney writing that it feels "so amazing" to have a baby.