The 40-year-old singer announced the news Tuesday on Twitter. The couple celebrate their third wedding anniversary July 15, a milestone Lachey acknowledged in his joyful post.
"Can't think of a better way to celebrate 3 years of marriage to my beautiful @VanessaLachey than this! #family," he wrote. He included a photo with the announcement, "It's a girl."
The couple's first child, Camden John, celebrates his second birthday in September. Vanessa shared details about her son with E! News in February, and hinted at expanding their family during the interview.
"The little man is out of this world," she said of Camden. "He's 18 months now and his personality is coming out. He's putting sentences together. He got his first haircut. I feel like next is college and girlfriends."
"I could go a minute with just the three of us right now," she continued. "But I was telling somebody earlier today that men have a biological clock. [Nick]'s like, 'Baby, I'm 40. We gotta get it moving."