The ''Four Weddings and a Funeral'' star used his Twitter account to announce Saturday: ''Am thrilled my daughter now has a brother. Adore them both to an uncool degree.''
He said both children ''have a fab mum'' and: ''To be crystal clear, I am the Daddy.''
The birth was confirmed by Grant's publicist, Carrie Gordon.
Grant's sometime girlfriend Tinglan Hong gave birth to the couple's daughter in 2011.