The 32-year-old yoga instructor took to Instagram to share the wonderful news.
"We are so pleased to introduce you to our new baby, Leonardo Ángel Charles Baldwin. It was such a special day bringing him into this world," Hilaria penned alongside a beautiful photo of her cradling her newborn in her arms.
Moments later, 58-year-old Alec re-posted the same snap on his Instagram page.
Leonardo was born in New York City on Monday.
The couple, who married in 2012, are already the proud parents to three-year-old daughter Carmen and one-year-old son Rafael.
The 30 Rock star announced they were expecting their third child together back in March with a loved up post of him and Hilaria sharing a tender kiss on the streets of New York, as the pregnant beauty held a blue teddy bear between them.
“Ireland, Carmen, Rafael, Hilaria and I are excited to share with you that we are going to have another little addition to our family. Another little boy coming this Fall,” he lovingly penned.
Alec shares 20-year-old daughter Ireland Baldwin with his ex-wife Kim Basigner.
Hilaria has garnered a cult following on Instagram thanks to her daily yoga posts.
Since welcoming her children into the world, Hilaria has even started to incorporate them into her routines.
"I wanted to do a pose each day. I've now realised that it's really hard to think up a new one every day! But I made a promise and I'm going to keep it. The best poses are the more spontaneous ones," she explained to Hello! Magazine.
Congrats to Alec and Hilaria on their newest addition!