The Oscar-winning Monster star is a first-time mum after taking in a "healthy baby boy named Jackson", according to her representative.
UsMagazine.com reports the tot was born in the U.S. and is of African-American descent.
A source tells the website, "She has always wanted to be a mum. She is glad to be able to do it on her own now and is so happy to be a mum."
Theron, 36, will begin parenting as a single mother - her nine-year relationship with actor Stuart Townsend ended in January, 2010.
But the South African-born beauty has made no secret of her desire to enter motherhood - shortly before splitting from Townsend, she said, "I feel very blessed to have had a certain amount of success and now I'm reaching the point where I would love to be a mum.
"I know I want to raise a big family and it's just a question of time - I know I'm ready for that phase of my life."