The announcement of the split by London law firm Schilling came as something of a surprise following the pair's portrayal as the ideal show business couple for many years.
Winslet, 34, who likes to refer to herself as "Kate from Reading" - outside London - married the successful British filmmaker and stage director in a Caribbean wedding in 2003.
They have one son, Joe, 6, while Winslet has a daughter, Mia, from her first marriage to film director Jim Threapleton.
"Kate and Sam are saddened to announce that they separated earlier this year," said a statement by Schillings.
The split was "entirely amicable and by mutual agreement" and both parties were fully committed to the future joint parenting of their children.
Winslet began her career as a teenager in BBC television, but shot to international stardom with her role as Rose Dewitt Bukater in the 1997 blockbuster Titanic.
Last year she won an Oscar for Best Actress for her highly acclaimed lead role in the German postwar drama The Reader. Other films include Enigma, Jude and Hamlet.
Mendes, 44, won a best director award for his movie American Beauty in 2000. The pair worked together on his fourth film, Revolutionary Road, in 2008.
"It was very exhilarating to finally work with my wife and I saw a side of her I just didn't know," he said of the experience.
