This is the couple's first child together. Arnett is also father to two sons from his former marriage.
Will Arnett is a dad for the third time. The Arrested Development star and his girlfriend, Alessandra Brawn, welcomed their first child together, a baby boy they named Alexander Denison Arnett, a rep for Arnett confirms to ET.
"Alexander Denison Arnett was born in Los Angeles on May 27th. 'Denny' is home, and everyone is doing well," the statement reads.
Us Weekly was first to report the news.
This is Arnett's third son. He is also father to two sons, Archie, 11, and Abel, 9, whom he shares with ex-wife Amy Poehler. Brawn, the former CEO of Chapel, also has a son from her previous marriage to Jon Neidich.
News of Brawn's pregnancy broke in February of this year, just months after she and Arnett made their red carpet debut as a couple at the 2019 Creative Arts Emmy Awards in September.
In 2017, the actor told ET about his parenting techniques and how he used his role in The LEGO Batman Movie to his advantage. Admitting that he sometimes leaned on his superhero voice to help parent his two sons, he said, "Just the kids when I'm trying to get them to turn off their iPads. It allows me to drop into the Batman voice when I need to drop the hammer at home."
Meanwhile, Arnett and Brawn join Iggy Azalea, who also recently gave birth to a baby boy. For more on the rapper's new baby, watch below.