The 39-year-old "Mad Max: Fury Road" star adopted the girl who was born in the United States in July, reported E! Online.
August will join her three-year-old elder brother, Jackson, who is also African-American and was adopted by Theron in 2012.
The actress had earlier opened up about the lengthy wait before she was allowed to bring Jackson home.
"My mom said the most beautiful thing... She said, 'It took me nine months to fall in love with you while you were growing in my stomach.' She's like, 'It took you two years to fall in love with this little baby.'
Theron and actor Sean Penn, 54, who recently ended their relationship, reunited earlier this week to re-shoot scenes for their movie "The Last Face".