Thomas, a redshirt junior, left Penn State for personal reasons on June 30, less than a month before the NCAA announced unprecedented sanctions against the program in the wake of the Jerry Sandusky child sex abuse scandal.
"We're excited about him, he's a great young guy and we're happy that he's a part of our program," Thundering Herd head coach Doc Holliday said. "He had an interest in playing for us, so we went through the proper procedures so that he can be here. We're glad he is."
Since Penn State is ineligible for postseason play as a result of its four-year bowl ban, Thomas can play this season for Marshall.
The 6-foot Thomas appeared in nine games last season for the Nittany Lions. He was expected to compete for a starting position this season and was listed as the No. 1 right cornerback on the team's initial 2012 depth chart.
Thomas is one of many players to leave Penn State during the school's tumultuous offseason.
Several players, including running back Silas Redd and wide receiver Justin Brown, decided to transfer after the NCAA announced Penn State's sanctions on July 23.
ESPN's Joe Schad contributed to this report.