The 57-year-old and his TV producer girlfriend are already parents to five-year-old John Mungo and another child, born in December 2015.
As well, the Notting Hill star shares six-year-old Tabitha and four-year-old Felix with ex Tinglan Hong.
Elizabeth Hurley spilt the beans on his and Anna's youngest child.
Speaking on E!'s Daily Pop, Elizabeth said of the Paddington 2 actor, "I'm very lucky that 31 years into our friendship, he's still my best friend in the world. He's a really great guy.
"I see him a lot; I speak to him a lot. You know, he's now a father of five; he has five kids and he's a great dad. Yeah, he will remain my best friend for life."
The actress, also an ex of Hugh's, shared more about the newborn on Watch What Happens Live With Andy Cohen.
"He had another one last week. He has five," the 52-year-old said of the Love Actually actor.
"He was over 50 when he spawned them all! He's an enchanting dad – really, really sweet. Having these kids has transformed him from a very miserable person into a fairly miserable person. It's improved him. He's gone up the scale."
Congrats to Anna and Hugh on their growing family!