The Titanic actress welcomed a son with husband Ned Rocknroll on Saturday, 7 December. The baby was born at an NHS hospital in Sussex, according to a rep.
He weighed 9lbs. No name has been announced yet.
A spokesperson told People that Kate and her new arrival are "doing great."
This is the first child for Kate and Ned. Kate is also mum to daughter Mia, 13, from her marriage to Jim Threapleton and 10-year-old son Joe from her marriage to Sam Mendes.
37-year-old Kate announced her pregnancy back in June.
She married 34-year-old Ned Rocknroll - the nephew of Sir Richard Branson - in a private ceremony in December 2012 following a summer engagement.
Her pregnancy announcement came a few days after a source told a US magazine the actress was hoping to get pregnant by the end of the year.
The insider told American Star: "Kate knows how much Ned wants to be a father. She thinks they would have the most adorable kid together."