The Love Actually star, who is rumoured to be dating Chinese actress Tinglan Hong, has become a father for the first time.
"I can confirm that Hugh Grant is the delighted father of a baby girl," the actor's rep Carrie Gordon told E! News.
"He and the mother had a fleeting affair and while this was not planned, Hugh could not be happier or more supportive. He and the mother have discussed everything and are on very friendly terms."
A source had earlier told Us Weekly that the actor welcomed a daughter in early October, adding that Grant was "thrilled" by the birth.
"He's going to be a great dad!" the insider said. "He's beaming about his daughter. He's not with the mum anymore but he's absolutely thrilled."
The 51-year-old confessed last year that he was "dreading" turning 50, joking that he would commit suicide on the special day just to avoid getting older.
Grant was taken to hospital in March after complaining of feeling faint, dizzy and short of breath.
The actor previously dated actress Elizabeth Hurley and British writer Jemima Kahn.