The 30-year-old actress and her boyfriend Tom Sturridge, 26, are expecting their first child together, Us Weekly has reported.
The couple - who have been together for several months - are said to be thrilled with the happy news but although they expecting a child they "don`t have plans right away to marry".
Friends of Sienna have given actor Tom the seal of approval and are delighted for the couple and "not surprised by the news of the pregnancy".
A source said: "Her friends are really happy with her relationship with Tom. He`s younger, yes, but he`s mature and he`s a good guy - They`re really good together. They just spent the holidays in Paris together after they announced the news in London."
Sienna started dating 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.
They initially separated in 2006 after he was revealed to have cheated on the `G.I. Joe: The Rise of Cobra` actress after he was revealed to have cheated on her with his children`s nanny.