Ciara Parkes said "The Talented Mr. Ripley" and "I (Heart) Huckabees" actor Law proposed to Miller, his blonde "Alfie" co-star, in England on Christmas morning, presenting her with a gold ring featuring nine diamonds set in platinum.
"Both their families and friends are thrilled and of course Jude's children are too," Parkes said.
Law, who turned 32 on Dec. 29, divorced fashion designer Sadie Frost in October 2003 after a six year marriage and three children.
Miller celebrated her 23rd birthday on Dec. 28.
A date for the wedding has not been set.