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."