The pair, who've been dating quietly for some time, enjoyed their first public outing together on Saturday, walking around London's trendy Soho and Mayfair districts.
"They're seeing each other," a friend of Winslet's tells PEOPLE. "She's known him since earlier this year." Friends point to the happiness she has shown in the photographs as proof of how she's feeling.
On Saturday, the Oscar winner, 34, was beaming bright as she clasped her date's hand after leaving the Groucho Club. Later, they were pictured leaving Mahiki, a nightclub popular with Prince William and Prince Harry.
Inside the tiki bar, they had sipped on the club's signature Treasure Chest cocktail and danced to some of the "fun music" – everything from Bananarama to Katy Perry – which a deejay spun, a source tells PEOPLE.
"They arrived just before midnight and left around 3 a.m.," a source at Mahiki says. "They were holding hands as they left to get in their car."
Neither Winslet's rep nor an agent at his model agency, Select, had a comment.