The couple, who married in 2011, welcomed their son Camden John in September 2012.
Nick revealed the news on Twitter where he posted a sweet snap of his son Camden below a heart in the sand and the words "it's a girl" in the middle.
Vanessa suffered from post-partum depression following Camden's birth and in 2013 she opened up on her suffering.
"My brain was working overtime. I had so many questions and needs and wants, but nothing I could ask of anybody visiting… Except maybe a mom", Vanessa wrote in a blog post.
Nick Lachey and Vanessa Minnillo in 2014
"I think it’s just fear…The fear of not knowing what I’m doing. The fear of “messing up” this little boy. The fear of being responsible for a human being and not knowing any “life” experiences to compare moments with him to", she added.
"I felt lost, unloved, alone and at my wits end"
After overcoming her fears though, Nick recently revealed that Vanessa now relishes being a mother. "My wife's maternal side is a new dynamic to her personality," Lachey told Parents magazine earlier this year. "It's definitely made her even sexier."
The couple regularly update their fans on social media, and Vanessa instagrammed an adorable snap of husband Nick and their son Camden on Father's Day.
Vanessa captioned the shot: "One of the greatest men I know... there's nothing you can't do, Baby. You are an amazing Friend, Husband and Daddy. We Love you forever and ever. Happy Father's Day!"