The Notting Hill and Bridget Jones star is "thrilled" to have welcomed a baby boy, a brother for his one-year-old daughter Tabitha, with Chinese actress Tinglan Hong - a relationship that started out as a "fleeting affair".
The 52-year-old film star has almost been more famous for his appearance at the Leveson Inquiry and his campaigning for a more responsible press of late than for his films, and his tweets are not without a reference to that passion.
He tweeted: "In answer to some journos. Am thrilled my daughter now has a brother. Adore them both to an uncool degree. They have a fab mum.
"And to be crystal clear. I am the Daddy.
"Thanks for v kind messages. Now I've confirmed (press somehow got birth certificate & were calling),hoping my family will be left in peace."
When Tabitha was born in 2011, Grant's publicist released a statement revealing the nature of his relationship with Ms Hong: "I can confirm that Hugh Grant is the delighted father of a baby girl.
"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."
Grant has had relationships with high-profile figures in the past - most notably actress Elizabeth Hurley and socialite Jemima Khan.
He managed to move on from the scandal that engulfed him following his well-publicised encounter with a Hollywood prostitute in 1995.
His previous film credits include About A Boy, Love Actually and Four Weddings And A Funeral.