"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."
Wide receiver Jadon Haselwood also announced Monday that he was entering the transfer portal.
Rattler started all 11 games as a redshirt freshman last season, throwing for 3,031 yards and 28 touchdowns with seven interceptions. He was the AP first-team and 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 Heismans 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 reentered the game and led OU to a victory. This season, true freshman Caleb Williams came in, rallied against 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 overall recruit in the 2019 ESPN 300, will have three years of eligibility remaining.
Haselwood was second on the team in receiving this season, with 39 catches for 399 yards and 6 TDs. He was the No. 12-ranked prospect overall, and the top-ranked receiver, in the 2019 class.