He told Kelly Ripa the news while serving as guest co-host of the morning program.
Vanessa is due to deliver late this summer or early fall, he said, adding they don't know the baby's gender yet.
"I've had a chance to do a lot of great things in my career and my life but this is the one thing that, more than any other, I look forward to. The fact that it's actually here, it's overwhelming," Nick Lachey said.
The couple married last July after dating for several years.