Johnson will graduate this June with a bachelor's degree in criminal justice from TCU. He is expected to arrive in Corvallis in June and will be available immediately as a graduate transfer.
The 6-foot, 237-pound back rushed for 789 yards and eight touchdowns in 36 games over his career at TCU, after redshirting as a freshman in 2013.
He ran for a career-best 105 yards in a game against Texas Tech in 2014.
"We are very excited to have someone of Trevorris' ability, character and focus join our program," Beavers coach Gary Andersen said. "He brings a level of maturity to our team on and off the field."
Johnson plans to pursue a master's degree in counseling at Oregon State.