The pair share two sons, Luca and Matteo, who were both born in Rome, and lived together between London and Italy.
A joint statement from their publicists confirmed the news on Friday.
It said: "Colin and Livia Firth have separated. They maintain a close friendship and remain united in their love for their children.
"They kindly ask for privacy. There will be no further comment."
The Academy Award, Golden Globe and Bafta winner shares a son with former partner Meg Tilly, his co-star in the film Valmont.
After they split Firth married Giuggioli in 1997.
Colin and Livia were often seen together at red carpet events.
Livia is an environmental activist and co-founder and creative director of Eco-Age, a sustainable consultancy firm.
She is also a founding member of singer Annie Lennox's women's advocacy group The Circle.
In 2017 it emerged that Firth had followed in the footsteps of his children and applied for dual UK and Italian citizenship, while Livia was applying for a British passport.
Firth is best known for playing a number of famously stiff-upper-lipped English figures on the big screen, including Bridget Jones's Mark Darcy, Love
Actually's Jamie, and King George VI in The King's Speech.