The 38-year-old actress, who married Ned Rocknroll in a secret ceremony last year, gave birth to their child at a hospital in Sussex County, England on Saturday.
“Mother and baby are doing great,” her rep said in a statement, though there's no information about the baby's name.
Winslet already has a 13-year-old daughter, Mia Honey, with ex-husband Jim Threapleton, and a 10-year-old son, Joe, with ex-husband Sam Mendes.
Though the new boy is Winslet's third child with three separate dads, the actress told Vogue in October that she has full custody of her two eldest.
"They've always been with me," she said. "They don't go from pillar to post; they're not flown here and there with nannies. That's never happened. ... My kids don't go back and forth; none of this 50/50 time with the mums and dads -- my children live with me; that is it. That is it!"
At the October premiere of her film Labor Day, a very pregnant Winslet said that she was too "absorbed in baby" to think much about awards season, though her dramatic role opposite Josh Brolin has generated Oscar buzz.