"Kate had 'Baby Boy Winslet' on Saturday at an NHS Hospital," her spokeswoman confirmed, adding that that the new parents are "thrilled to bits".
It's the first child for 35-year-old Ned, who is the nephew of the star's friend Virgin mogul Richard Branson.
Kate, 38, already has two children – 12-year-old Mia, whose father is director Jim Threapleton and Joe, nine, from her marriage to filmmaker Sam Mendes.
Earlier this year she responded to criticism of her family life.
"[My children have] always been with me," Kate revealed. "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!
"People go, 'Oh, my God! Those poor children! They must have gone through so much.' Says who?," she added.
Kate and Ned began dating in 2011 after meeting at Sir Richard's idyllic Caribbean hideaway, Necker Island. She was clearly attracted to his fun-loving spirit.
Formerly Ned Abel Smith, he changed his name by deed poll in 2008 to RocknRoll, "as a fun reminder to make sure every moment of every day is spent wisely and enjoying life".
Their union came a year after Ned, who works at Virgin Galatic – his uncle's space travel project – split from his wife, aristocrat Eliza Cowdray.
Despite being head over heels in love, Kate has said she's going to stick with her surname. "I was never going to change my name to Rocknroll," she said. "I've never changed my name to anything so I didn’t see a reason to start now."