The "Sin City: A Dame To Kill For" star shared the news on Instagram, posting the first photo of her baby boy, Hayes.
"Hayes Alba Warren. Best gift to ring in the New Year!!" wrote the actress.
"Cash and I feel so blessed. Haven and Honor are already obsessed with their new baby bro," she added.
Alba also has two daughters, nine-year-old Honor and six-year-old Haven, with her husband.
Warren also posted a photo with his newborn son on Instagram and wrote, "Hayes Alba Warren: You sure know how to ring in the new year! You showed up a few days early but we couldn't be any happier. Your Mamma is the strongest woman I know... you're so lucky to have her by your side."