"So excited to introduce Marvel Jane Wentz. Feeling so grateful that she’s here... annnnd she’s already ready to takeover the world," the 38-year-old musician wrote, alongside the cute family photo, which also included their 3-year-old son Saint Laszlo and his 9-year-old son, Bronx Mowgli, whom he shares with ex Ashlee Simpson.
"Happy Mother’s Day @meagancamper and all the moms out there," Wentz added. "We [heart] you."
Wentz announced that he and his girlfriend were expecting their second child together on New Year's Day, with an Instagram pic of his two sons sitting on the front steps of his house while holding a beautifully wrapped pink present.
"Happy New Year! We’re kicking the year off with news of the best gift yet," Wentz wrote.
Wentz and Camper, 28, began dating in 2011 and welcomed Saint in 2014. Congrats to the happy family!
In January, ET caught up with Wentz and his Fall Out Boys bandmates at their iHeartRadio album release party at the iHeartRadio Theater in Los Angeles, and he opened up about how rocking out with other musicians like Justin Bieber and Jaden Smith earned him "ultimate dad points" from his sons.