Firth took up Italian citizenship on Friday, the interior ministry in Rome announced.
"The celebrated actor, who received an Oscar for his work in the film 'The King's Speech,' has married a citizen of our nation and has repeatedly expressed his love for our country," it said in a statement.
Firth, 57, married Italian producer Livia Giuggioli, 48, in 1997, according to the specialist movie site IMDb. They have two children.
His many successes include "Pride and Prejudice", "Love Actually", "Kingsman: The Secret Service" and the movie adaptation of the stage show of ABBA hits, "Mamma Mia!"