Hefner wrote on Instagram: “At 5:23pm this afternoon Scarlett and I welcomed our daughter Betsy Rose Hefner into the world. We are overwhelmed with gratitude, love and joy.”
He shared pictures of the new baby and a video of Byrne, who was wearing a face mask pulled down, sharing a proud kiss.
The couple were flooded with messages of congratulations from the celebrity friends.
Byrne’s Harry Potter co-star Evanna Lynch, who played Luna Lovegood in the film franchise, commented: “Oh my goodness she’s perfecccct!!! congratulations!!!.”
Hefner and Byrne – who have been dating since August 2015 – tied the knot at a registry office in Ventura, California in November 2019. Byrne posed nude for Playboy magazine in February 2017.
They announced they were expecting their first child in March.
Byrne wrote on social media: “Cooper and I are delighted to share that we have a little one on the way. Our hearts are filled with gratitude and joy, and the two of us can’t wait to meet the newest little Hefner.”
Cooper is the youngest of the late Hugh Hefner’s four children. He has a brother Marston, who is just one year older than him, who the Playboy mogul fathered with second wife Kimberley Conrad.
He also has a half-sister Christie, 66, and 64-year-old half-brother David, who Hef had with his first wife Millie Williams.
Cooper is a businessman and was previously the chief creative officer of Playboy Enterprises. In July he announced he is running as a Democrat for the California Senate.
Hugh Hefner died in 2017 aged 91, leaving behind an estimated $50m fortune and a business empire based around the Playboy brand.