The 38-year-old, who married Ned Rock'n'Roll after a whirlwind romance last December, is said to be overjoyed.
A spokesperson for the actress confirmed:
'Kate had ‘Baby Boy Winslet’ on Saturday at an NHS Hospital,' adding that that the new mother and husband Ned Rocknroll were 'thrilled to bits.'
This will be Kate's third child, but first for her husband.
Kate is also mother to Mia, 13, and Joe, 9, from her first two marriages.
'I was never going to change my name to Rocknroll'
The Titanic star's husband, who works for Richard Branson's company Virgin, was born Ned Abel Smith, but changed his name by deed poll.
However Kate has previously confirmed she won't be taking his unusual surname.
'I was never going to change my name to Rocknroll. I've never changed my name to anything so I didn’t see a reason to start now.'
Congratulations guys!