The 38-year-old actress Instagrammed an adorable photo of the couple snuggling with their newborn, who was born on Dec. 20.
"@Joshbkelley and I are beyond thrilled to finally share the arrival of our little man Joshua Bishop Kelley Jr. born December 20th, weighing in at 7 pounds 15 ounces," Heigl wrote. "We are all happy, healthy and beyond grateful!"
She also thanked all her fans for their support.
"And one more GIANT thank you to all of you for your wonderful words of support, love and all those beautiful well wishes!" she added. "As always I am incredibly grateful to have such wonderful fans!"
Joshua Jr. joins older sisters Naleigh, 8, and Adalaide, 4, in the family of five. Heigl revealed how she shared the news that she was pregnant with Naleigh and Adalaide -- whom she adopted in 2009 and 2012, respectively -- in a new interview with People.
"[The news] was a little confusing for the girls because they'd be like, 'I grew in your tummy?' And I had to explain, 'Well, you didn't actually grow in my tummy, but your dad and I are your parents,'" she shares.
The Heigl family is no stranger to adoption. Heigl's older sister, Meg, was adopted from Korea.
"My mother used to say this too: 'It doesn't matter how they come into your life,'" the former Grey's Anatomy star tells the magazine. "'The moment they put that child into your arms -- whether you just [gave birth] or whether they just arrived from Korea -- they're yours.'"
"Usually I'm pretty good at memorizing," she explained. "I can do that pretty quickly. It's like the only thing I've got. So, it's really been eye-opening to me to struggle with that."