Fortt, who will be immediately eligible to play this season, becomes the sixth Nittany Lion player or recruit to go elsewhere without penalty since NCAA sanctions were handed down in the Jerry Sandusky child abuse case.
The 6-foot-3, 238-pound Fortt gives the Bears another option inside as they look to replace incumbents Mychal Kendricks and D.J. Holt. However, he is still recovering from knee surgery that kept him out of spring practice. Fortt could redshirt this season and still have two years of eligibility remaining.
In two seasons in Happy Valley, Fortt quickly emerged as a key reserve, playing in 10 games as a true freshman. He had 33 tackles, six tackles for loss and 2.5 sacks last season, and was expected to compete for the open middle linebacker job.
"The friends I've made here (at Penn State) are friends for life," Fortt told the Stamford (Conn.) Advocate. "Some people have different paths."
The arrival of Fortt comes after sophomore Cecil Whiteside was reinstated Monday. Whiteside had been dismissed in May for a violation of team rules.
Whiteside, who had four tackles for loss, three sacks, and two forced fumbles in 10 games last season, was listed as a backup outside linebacker on the post-spring depth chart.
Dan Greenspan is the publisher of Cal Sports Digest and covers the Pac-12 for Fox Sports/Scout.com. Follow him on Twitter @DanGreenspan.