The bassist will join The Muscular Dystrophy Association's 2014 National Goodwill Ambassador Reagan Imhoff on stage following a performance of the band's new song Young Volcanoes on Sunday (31Aug14).
He says, "Reagan is a total sweetheart whose positive attitude immediately captures the attention of everyone she meets.
"Being able to give back to an organization that's making a difference like Mda has taken on a much deeper meaning for me since having kids. All the members of Fall Out Boy are honoured to be part of this telethon as a band to help save and improve lives for all Mda families."
LeAnn Rimes and actress Alyssa Milano will also attend the televised event, which will help raise money for the muscular disorder.
The telethon will air in the U.S. on Sunday.