The Titanic actress and businessman husband Ned Rocknroll chose the unusual moniker for her third child, born in December 2013.
Chatting on The Ellen DeGeneres Show this week, Kate revealed the family – including children Mia and Joe, from previous marriages – had chosen the name Bear early on in her pregnancy.
"When I was pregnant with Mia and Joe I didn't name them before they came out because I always felt I needed to see them and who they're going to be," she said.
"This time around there was Mia and Joe and the whole pregnancy was about what we're going to call the baby. So it was really lovely, actually, as they were very much included. We settled on Bear very early on."
As for the inspiration behind the unique name, she said: "A friend of mine when I was younger was nicknamed Bear, and I just had always really loved it. He was very much a bear. He was everyone's shoulder to cry on, he was a big bear hug, he was just this great figure in my life, and I just always remembered him, so that's where it came from."
Baby Bear's middle name is Blaze, a reference to the house fire on Sir Richard Branson's Necker Island, which is where Kate and Ned – Richard's nephew – met for the first time.
"We wanted something of the fire so that's the name we came up with," said Kate.
And she reiterated that 14-week-old Bear Blaze has been given her surname, Winslet, rather than his father's name of Rocknroll.
Divergent actress Kate was in LA for just 48 to accept a star on the Hollywood Walk of Fame, but it was certainly an eventful visit as she experienced her very first earthquake yesterday.
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Watch Kate chatting about the earthquake and her baby's name below:
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