Welcome to the world, Lyla Maria Schwarzenegger Pratt!
"We are beyond thrilled to announce the birth of our daughter, Lyla Maria Schwarzenegger Pratt," they wrote. "We couldn’t be happier. Both mom and baby are doing great. We are extremely blessed. Love Katherine and Chris."
Pratt included a pair of psalms in the caption, including Psalm 126:3: "The LORD has done great things for us, and we are filled with joy."
Lyla is the couple's first child. Pratt has a son, Jack, 7, with ex-wife Anna Faris.
Anticipation had built for the first glimpse of the baby girl after Schwarzenegger's brother Patrick told "Entertainment Tonight" on Sunday about the happy news.
"They're doing great — just got her a little gift," he told ET, although it wasn't clear if he was referring to his sister or the baby.
The couple did not share how they came up with Lyla's name, but her middle name, Maria, is the first name of Schwarzenegger's mother, TODAY contributor Maria Shriver. She spoke on TODAY last month about the excitement of becoming a grandmother.
"I've started to get really excited, actually, about it," Shriver told Hoda Kotb. "This last 4 1/2 months, I think, I've gone like everybody else through this whole wave of emotions. You've been like, 'OK I can manage this, it's gonna be over in a little bit.' Then you get depressed, and you're like, 'My God, what's happening?'
"And this has been kind of a little light there, right? It's something to look forward to when we don't really have much to look forward to."