The rock star took to Instagram to share the exciting news with his 1.5m followers, while wishing them all a Happy New Year.
Sharing a photo of his two sons, Bronx and Saint - who was holding a little pink present box indicating that they're expecting a girl - Pete captioned the photo: 'Happy New Year! We're kicking the year off with news of the best gift yet: coming to our family in 2018... love Pete, Meagan, Bronx and Saint.'
Hundreds of congratulatory comments followed, with many of his fans commenting how happy they are for the pair.
The showbiz couple, who have been together since 2011, already have three-year-old son, Saint Lazlow together while Pete's eldest son, nine-year-old Bronx Mowgli is from his former marriage with his ex-wife Ashlee Simpson.
As this new addition will be Pete and Meagan's second child together, their baby girl will be a great new addition to their growing family. So far the pair haven't announced when their baby is due.
It's no wonder that Pete was excited to share the news with his fans, as he has previously described that being a father has particularly helped him both mentally and emotionally with his biopolar disorder.
Last year, the 36-year old band member talked to The Red Bulletin and described his views on fatherhood as being 'pretty much like taking happy pills'.
He explained: 'My highs, my happiness, are really high, and my lows are very low, and I'm not able to regulate between the two.
'Through actual therapy and having kids, it's way more under control and something I can see when I'm on the roller coaster and control it more.'