Wentz, a bassist and lyricist known for his band "Black Cards" and for his time with "Fall Out Boy," also has a 5-year-old son named Bronx Mowgli with his ex-wife Ashlee Simpson.
"We're super excited to announce we're expecting a baby," Wentz wrote in his online message, announcing Camper's pregnancy.
He did not say what the baby's gender or anticipated due date are.
"They've been planning on this for a while. And they're really excited for Bronx to have a new little buddy," a source told People.com. "Meagan is a big sister and has a great relationship with Bronx. She's naturally maternal so this is perfect."
Millions of Getty images now available for free via embed tool
Boston schools pull out free condoms over wrapping complaints