The Hollywood pair told aides they want to set up home in London in the spring - three years after splitting up.
Speculation had been rife about whether Jude, 36, and Sienna, 27, had rekindled their romance while they were both appearing in plays on Broadway, New York.
But The Sun can reveal they ARE back together - and it's serious. They have been meeting up at the Algonquin Hotel in New York during rehearsal breaks - Jude in Hamlet and Sienna in After Miss Julie - and things have blossomed.
A source said: "Last week they put the wheels in motion to live together again.
"They have told staff that they are in love and will be moving back to London in the spring and they will be living together as a couple once again."
The pair, who have also been spotted outside his New York apartment, began their romance after meeting on the set of Alfie in 2004. Sienna dumped Jude in 2005 after he cheated on her with his children's nanny.
They got back together before splitting up again in November 2006.
It is Sienna who has been the most keen to get back together.
The source added: "Jude was reluctant at first for anything to happen. But Sienna convinced him that she was still in love with him."
Since their parting in 2005 they've had complex love lives.
She recently split from actor Balthazar Getty who had briefly left his wife and four kids for her. Jude has just fathered his fourth child, the result of a one-night stand with model Samantha Burke.
The pair met up again through mutual friend Jonny Lee Miller.
Jude's next film, Sherlock Holmes, opens in cinemas on Boxing Day.