The rock star shared his happy baby news via Instagram on Monday night (17 February), telling fans: "we're super excited to announce we're expecting a baby!"
Pete added a picture of himself and Meagan lying on a bed together kissing, with his hand pressed against her stomach.
'My Songs Know What You Did In The Dark' star Pete is already a dad to son Bronx, five, born during his marriage to Ashlee Simpson.
Bass player Pete and model Meagan have been dating for several years.
Pete recently said on US TV that he's still amazed brunette Meagan is with him, joking when presented with a picture of them together: "I'm looking at this picture and I'm like, 'Wow, it's insane that she hangs out with me.'"
We're sure plenty of Fall Out Boy fans would disagree!
Pete, 34, added in the same interview on The Wendy Williams Show that he and Meagan had been discussing marriage, describing his girlfriend as his best friend and confidante.
Meanwhile, it's been a happy year so far for Pete's ex Ashlee as well, who announced in January that she's engaged.
Singer and ex-reality star Ashlee accepted a proposal from boyfriend Evan Ross, the son of singer Diana. He's an actor who will appear in the next Hunger Games movie.
More celebrities who have announced pregnancies in 2014 below!
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