The 57-year-old actor has been married to Italian producer and environmentalist Livia Giuggioli for 20 years and the couple live in London with their two sons.
The Italian interior ministry said it had granted citizenship to a person “married to a citizen of our country and who has declared his love for our land on several occasions”.
Firth, who remains a British national, obtained Italian citizenship after Brexit, which has created uncertainty for EU citizens living in Britain and British nationals residing in the EU.
“My wife and I are both extremely proud of our own countries. We feel that we’ve made a gift of that to each other,” Firth said in a statement.
“Our children have been dual citizens since the beginning. We never really thought much about our different passports. But now, with some of the uncertainty around, we thought it sensible that we should all get the same. Livia is applying for a British passport.
“I will always be extremely British (you only have to look at or listen to me). Britain is our home and we love it here. Despite the enticements of my profession to relocate to more remunerative climes I’ve always chosen to base my career out of the UK and pay my taxes here. That hasn’t changed."
With ANSA