The 30-year-old actress and her fiance Tom Sturridge, 26, welcomed their first child into the world over the weekend in London and the gender of the tot has now been revealed.
A source has confirmed the baby`s sex to UsMagazine.com but details about the child`s weight and name are still unavailable.
Although news of Sienna`s pregnancy broke earlier this year, the `G.I. Joe: Rise of Cobra` star was very reluctant to talk about it.
The blonde beauty admitted she did not want to share details of the pregnancy because she didn`t want anything to jeopardise her attempts to keep her private life out of the public domain.
In March, she said: "I`m in my second trimester, so I`m I-can-talk-about-it pregnant, even though I`m under strict instructions not to say anything as it would completely defeat the point of everything I`ve tried to achieve in the last eight years."
Sienna started dating `The Boat That Rocked` actor Tom - who is friends with Robert Pattinson - just a few months after she split from fiance Jude Law for a second time in February 2011.