Theron, 39, worked with an adoption agency for six months before adopting the African-American girl in July.
August joins big brother Jackson, 3, who Theron adopted from her native South Africa in 2012.
Charlize Theron/Sean Penn
Charlize Theron adopts second child, a baby girl -- find out her name!
HOLLYWOOD, CA - FEBRUARY 29: (HOLLYWOOD REPORTER AND US TABS OUT) Actor Sean Penn poses with his Oscar for Best Performance by an Actor in a Leading Role next to Charlize Theron and her Oscar for Best Performance by an Actress in a Leading Role during the 76th Annual Academy Awards at the Kodak Theater on February 29, 2004 in Hollywood, California. (Photo by Frank Micelotta/Getty Images)
LOS ANGELES, CA - APRIL 18: Actress Charlize Theron and actor Sean Penn attend the Points of Light generationOn Block Party on April 18, 2015 in Los Angeles, California. (Photo by Michael Kovac/Getty Images for generationOn)
LOS ANGELES, CA - FEBRUARY 27: Sean Penn and Charlize Theron pose with Pharrell Williams at Bionic Yarn, G-Star and Hennessy Privilege Celebrate Pharrell Williams and his Oscar nominated single, 'Happy' on February 27, 2014 in Los Angeles, California. (Photo by Dimitrios Kambouris/Getty Images for One View)
LOS ANGELES, CA - JANUARY 10: Actor Sean Penn and actress Charlize Theron attend the 'Help Haiti Home' gala at Montage Hotel on January 10, 2015 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)
LOS ANGELES, CA - JANUARY 10: Charlize Theron, Founder and CEO of J/P Haitian Relief Organization Sean Penn and Former President of the United States Bill Clinton attend the 4th Annual Sean Penn & Friends HELP HAITI HOME Gala Benefiting J/P Haitian Relief Organization on January 10, 2015 in Los Angeles, California. (Photo by Christopher Polk/Getty Images for J/P Haitian Relief Organization)
PARIS, FRANCE - JULY 07: (L-R) Sean Penn and Charlize Theron attend the Christian Dior show as part of Paris Fashion Week - Haute Couture Fall/Winter 2014-2015 on July 7, 2014 in Paris, France. (Photo by Pascal Le Segretain/Getty Images)
Charlize Theron and Sean Penn attend the world premiere of Universal Pictures 'A Million Ways To Die In The West,' at the Regency Village Theatre in Los Angeles, California, May 15, 2014. AFP PHOTO / ROBYN BECK (Photo credit should read ROBYN BECK/AFP/Getty Images)
WESTWOOD, CA - MAY 15: Actress Charlize Theron and actor Sean Penn arrive at the Los Angeles Premiere 'A Million Ways To Die In The West' on May 15, 2014 in Westwood, California. (Photo by Jon Kopaloff/FilmMagic)
HOLLYWOOD, CA - MAY 07: Actors Sean Penn (L) and Charlize Theron attend the premiere of Warner Bros. Pictures' 'Mad Max: Fury Road' at TCL Chinese Theatre on May 7, 2015 in Hollywood, California. (Photo by Kevin Winter/Getty Images)
PARIS, FRANCE - JULY 07: Sean Penn and Charlize Theron attend the Christian Dior show as part of Paris Fashion Week - Haute Couture Fall/Winter 2014-2015 on July 7, 2014 in Paris, France. (Photo by Dominique Charriau/WireImage)
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Being a mom comes naturally for Theron, who knew she wanted to adopt from an early age. Last year, the Oscar winner reminisced to Vogue about a letter she wrote when she was 8 that read, "What I want for Christmas this year is for you to take me to the orphanage so that we can adopt a brother or a sister."