Rosen made the expected announcement Wednesday with a post on Twitter. He is expected to be a high first-round pick in April.
After arriving in Westwood as the top quarterback recruit in the nation, Rosen was the Bruins' starter for the past three seasons, playing in 30 total games while missing half of his sophomore year with injuries.
He passed for 9,301 yards with 59 touchdowns and 26 interceptions, showing off the tantalizing talent that never translated into a Pac-12 title.
Rosen wrote that attending UCLA was "the best decision of my life."
"Over the last three years, UCLA has helped me grow as an athlete, a scholar and a member of the community," Rosen added. "I have made some mistakes along the way; however, I am grateful that I made those mistakes backed by such a supportive and positive university, so that I could learn from them and better myself."
Rosen played for three offensive coordinators in three seasons with the Bruins, but still developed the arm strength and polish that could make him the first overall pick.
He was close to fired UCLA coach Jim Mora, but also discussed his decision with new Bruins coach Chip Kelly. Rosen thanked his teammates during three seasons in which the Bruins went 18-20 and lost their two bowl games.
"I wish we had won more games, but friendships were forged and memories were made that will last a lifetime," he wrote. "It pains me to leave these guys, but I know they wouldn't let me come back and pass up this opportunity to chase my dreams."
He also promises his mother that he'll return to Westwood to finish his degree.
About 25 minutes after Rosen declared his intentions, Southern California quarterback Sam Darnold also announced he will give up his final two years of collegiate eligibility to head to the NFL.
Darnold's Trojans beat Rosen's Bruins 28-23 for the Victory Bell on Nov. 18 in the local products' only collegiate game against each other. Darnold redshirted the 2015 season, and Rosen was injured for the crosstown showdown in 2016.
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