"Sooner Nation: Thank you for allowing me to be a student-athlete at this prestigious institution," he wrote, while also thanking teammates and coaches. "We won several games together and made memories that I will cherish forever."
"Thank you again, Oklahoma," he concluded. "I will miss you."
Rattler started all 11 games as a redshirt freshman last season, throwing for 3,031 yards, 28 touchdowns with seven interceptions and was the first team All-Big 12 quarterback.
Coming into this season, he was a heavy favorite for the Heisman, following in the footsteps of Baker Mayfield and Kyler Murray, who won Heisman Trophies under Lincoln Riley, and Jalen Hurts, who was a Heisman finalist with Rattler as a backup.
But Rattler was benched for the second straight year during the Sooners' rivalry game with Texas.
Last year, he re-entered the game and led OU to a victory. This season, true freshman Caleb Williams came in, rallied Texas to a thrilling 21-point comeback and kept a hold on the starting job.
Rattler, an Arizona native who was the No. 1 quarterback prospect and No. 29 over recruit in the 2019 ESPN 300, will have three years of eligibility remaining.
Riley accepted the head coaching at USC on Sunday, and the Sooners have lost several commitments since.