The baby joins Theron's first child, Jackson, who was adopted in South Africa in 2012, according to celebrity news website TMZ.
The 39-year-old American actress reportedly adopted US-born August in July, TMZ said.
Theron and actor Sean Pean recently called off their engagement, following reports that Penn planned to formally adopt Jackson.
The critically acclaimed actress won a Best Actress Oscar for her role as a prostitute-cum-serial killer in the grisly 2003 film Monster.
She is also known for her roles in The Devil's Advocate, The Cider House Rules and Mad Max: Fury Road. – Rappler.com