The actress made the announcement by sharing a short video of her self and her two daughters, Honor Marie, 9, and Haven Garner, 6, opening up a box filed with bright blue balloons and confetti signaling that her next child will be a boy.
"@cash_warren and I couldn't be more thrilled to announce... #officiallyoutnumbered #babyboy #cantwaittospoilhim #hugsandkissesforlife #soontobemamaofthree," Alba captioned the clip.
Alba announced in July that she was pregnant with her third child while in vacation in Hawaii.
Alba, 36, and Warren, 38, tied the knot in 2008.
The Honest Company founder celebrated daughter Honor's 9th birthday by sharing a loving message to her on Instagram. "I'm so proud of how compassionate, intelligent and thoughtful you are. The day you were born, I found my true calling in life. You turned my life and world right side up," she wrote at the time.