Pete Wentz and longtime girlfriend Meagan Camper announced their second child's arrival on Mother's Day this year

Over the weekend, Fall Out Boy bassist Pete Wentz couldn’t resist sharing a precious new photo of his baby girl, whose arrival he announced on May 13.

Little Marvel gazes up at the camera in the sweet photograph, dressed in a white onesie with a flower print, white gloves and a pink head wrap.

“MJ ❤️,” the proud father of three captioned his post.

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Wentz, 38, and longtime girlfriend Meagan Camper‘s daughter joined a household that also includes the couple’s son Saint Lazslo, 4 next month, and Wentz’s 9½-year-old son Bronx Mowgli from his previous marriage to Ashlee Simpson.

“We are so happy to welcome Marvel Jane Wentz into the world,” the rocker and model Camper, 29, told PEOPLE exclusively in a May statement. “Her brothers are both very excited about their brand new kid sister.”

He also shared the happy news on Instagram, writing, “So excited to introduce Marvel Jane Wentz. Feeling so grateful that she’s here … annnnd she’s already ready to takeover the world ❤🌎 happy Mother’s Day @meagancamper and all the moms out there. We ❤ you.”

RELATED VIDEO: Baby Girl on the Way for Fall Out Boy’s Pete Wentz

Wentz and Camper — who began dating in 2011 — announced they were expecting a daughter on New Year’s Day and opened up to PEOPLE about their expanding family in January.

“We have a lot of testosterone in our house,” Wentz previously told PEOPLE. “I see my friends who have daughters, and it seems like a whole different kind of love that they have for each other, so I look forward to that being a completely new journey.”

Fall Out Boy is set to perform on Late Night with Seth Meyers this Wednesday and in Good Morning America‘s Summer Concert Series on Friday. The band kicks off its 25-plus city nationwide M A N I A tour Aug. 29 in Uniondale, New York.