The 34-year-old star separated from Mendes at an unspecified point earlier this year, the British Shillings law firm said Monday. It wasn't clear whether divorce proceedings have begun.
In a statement e-mailed to journalists, the law firm said the split was "entirely amicable and is by mutual agreement."
"Both parties are fully committed to the future joint parenting of their children," the statement said.
The pair have been married since 2003 and have one child, son Joe Alfie. Winslet also has daughter Mia Honey from her first marriage to director Jim Threapleton, whom she divorced in 2001.
A call and an e-mail seeking further comment were not immediately returned.
Winslet won the Academy Award for best actress for her dramatic role in "The Reader" in 2009. It was the first Oscar win for Winslet, who had been nominated five other times for her roles in "Titanic," "Sense and Sensibility," "Iris," "Eternal Sunshine of the Spotless Mind" and "Little Children."
Mendes, an acclaimed stage and film director, won the coveted best director Oscar for "American Beauty."