The actor welcomed his first child on Tuesday.
“I can confirm that Hugh Grant is the delighted father of a baby girl,” his rep said in a statement.
“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.”
The identity of the mother is yet unknown. Earlier this year, the actor was linked to Chinese actress Tinglan Hong.
Grant, 51, had been in a 13-year relationship with actress Elizabeth Hurley before splitting in 2000. He is currently shooting the movie Cloud Atlas alongside Tom Hanks, Hugo Weaving and Susan Sarandon.