Publicist Laura Symons says Winslet had a boy, her third child, on Saturday at a hospital in southern England. She says mother and baby are both doing well.
The baby's name has not been announced.
This is 38-year-old Winslet's first child with husband Ned Rocknroll, whom she married last year. She has a daughter, Mia, and a son, Joe, from previous marriages to directors Jim Threapleton and Sam Mendes.
Rocknroll is the nephew of entrepreneur Richard Branson.
Winslet won an Academy Award for best actress in 2008 for her performance in "The Reader."