The couple, who are already parents to 22-month-old daughter James, have remained tight-lipped about the new arrival, but a source has shared the happy news to PEOPLE magazine.
There is no news yet about the baby's gender, or Blake and Ryan's choice of baby name.
The pair are already parents to daughter James, 22 months
In April 2016, a source revealed to the same mag that Blake, 29, was pregnant - news which she soon confirmed by revealing her baby bump at the premiere of her latest film, Café Society, at the 2016 Cannes Film Festival in May.
Since then, the actress has been sharing plenty of updates on her social media accounts, including this comedic snap, which she posted to her 11.5 million Instagram followers.
'Milkshake: check. Comfortable shoes: check. Having a slit in your dress to make you feel better about the aforementioned: check,' Blake joked of her comfy maternity look.
The new arrival makes the Reynolds a family of four, but it doesn't look like they'll be stopping there - Blake revealed on the TODAY Show back in June that both she and Ryan come from big families, and hope to follow in their parents' footsteps.
'I’m one of five kids. My husband is one of four, so we’re officially breeders,' she quipped. 'You can go on our website and we will give you some of our children.'
Ryan has also previously told PEOPLE that he doesn't mind whether his brood is made up of boys or girls.
'If I could have non-daughters I would be thrilled. Really! I genuinely would be,' he said. 'When I was a kid, my dad, he was a real strict guy. My brothers and I, we would destroy the house. That's part of the reason I don't want boys ... I just want the house to be normal.'
'I would love to have a little boy at some point, but I would not be an unhappy man if I had a six-person soccer team of girls'.