Marshall has agreed to join the Sun Belt Conference, as the Thundering Herd become the third Conference USA school to make the jump in the past week.
Marshall, along with Southern Miss and Old Dominion, will join the Sun Belt no later than July 1, 2023. Marshall had been a member of Conference USA since 2005.
"This is another big day for our conference," Sun Belt commissioner Keith Gill said in a statement. "Marshall's football tradition and passionate fans bring a strong presence to the Sun Belt. Their natural rivals are in our footprint and make our East Division even more competitive than it already was.
Also on Friday, James Madison -- an FCS powerhouse that won the national title in 2016 and has made two title-game appearances since -- took steps to join the Sun Belt. The school's board of visitors executive committee passed a motion to pursue moving up to the FBS level and seek an invitation from the conference.
The Sun Belt would reach 16 schools and 14 football programs if all the additions go through. It is focusing its expansion push on strong football programs within or near the league's footprint, rather than those in large metropolitan areas.
Following the departures to the Sun Belt and the planned moves of six other schools to the American Athletic Conference, Conference USA will be left with only five members: Western Kentucky, Middle Tennessee, Florida International, UTEP and Louisiana Tech. Conference USA commissioner Judy MacLeod has said the league is pursuing new members, including FCS schools that are considering making the move to FBS.
Other possible options for Conference USA include FBS independents Connecticut, Massachusetts, Liberty and New Mexico State.
UConn athletic director David Benedict told CTInsider.com that the school had an exploratory conversation with Conference USA officials about a football-only membership. UConn rejoined the Big East for basketball and most of its other sports programs last year.
ESPN's Adam Rittenberg and The Associated Press contributed to this report.