One of the top recruits of the 2015 class, Rosen decided to stay close to home to play his ball in Westwood. He was a first-team USA Today All-American as a high school senior, throwing for 3,186 yards and 29 touchdowns while his St. John Bosco squad won the California State Championship. UCLA coaches saw enough of his talent in the spring of 2015 (he graduated from high school one semester early) that they made him the first Bruin freshman opening-weekend starter ever. He won Pac-12 Freshman of the Year and several Freshman All-American honors after setting several school records and completing 60 percent (292-487) of his passes for 3,668 yards and 23 touchdowns (11 interceptions). Despite being only a freshman, he was in command of the offense and used his NFL size and arm to sling the ball all over the field. Rosen's sophomore season had its peaks (400 passing yards against Arizona State) and valleys (three interceptions vs. Texas A&M), and eventually ended with a season-ending injury to his throwing shoulder after six starts (59.3 completion percentage, 1,915 yards, 10 touchdowns, five interceptions). The surgery to his shoulder was not considered major, so he was throwing effectively in spring 2017 practices. Rosen had the performance of the year in the season's opening weekend, leading the Bruins in an amazing 45-44 comeback win with 491 passing yards and four touchdowns. He struggled over a four-game stretch at the start of the Pac-12 season, throwing eight of his 10 interceptions on the year. He did earn second-team All-Pac-12 notice, however, by completing 62.6 percent of his passes (283-452), throwing 26 touchdowns, and ranking second in the country with 341.5 passing yards a game (3,756 total). Rosen also suffered two concussions during the year, missing one regular season contest and their bowl game against Kansas State.