Law has finished filming Sherlock Holmes with Guy Ritchie
American model Samantha Burke is the woman carrying Jude Law's fourth child, the actor's spokeswoman has confirmed.
In a statement, a lawyer for Ms Burke confirmed the 24-year-old had a relationship with the star and was pregnant with his child.
It added Law, 36, had been "nothing but responsive and supportive" to the aspiring actress.
Law has three other children with ex-wife, actress Sadie Frost whom he divorced in 2003.
"Ms Burke can confirm that she did in fact have a relationship with Mr Law and that she has informed Mr Law that she is expecting his child later this fall," the statement said.
"Since informing Mr Law of the pregnancy, he has been nothing but responsive and supportive of Ms Burke and the pregnancy."
Law said he planned to play a "fully supportive" role in the child's life, despite no longer being involved with the mother-to-be.
The actor is currently playing Hamlet in the West End
It is thought the pair had a brief relationship while Law was filming Guy Ritchie's version of Sherlock Holmes in New York over Christmas.
Law earned Oscar nominations for his roles in 1999 thriller The Talented Mr Ripley and 2003 drama Cold Mountain.
He is currently playing the lead role in Hamlet in London's West End and will next be seen on the big screen in October in Terry Gilliam's The Imaginarium of Doctor Parnassus.
Sherlock Holmes, in which Law plays Dr Watson, is expected to be released on Christmas Day.