Rosen began taking classes at UCLA in January 2015; he had enrolled early so he could take part in spring practice. With three-year starting quarterback Brett Hundley leaving UCLA early for the National Football League (NFL), a large hole was created on the team. Rosen was UCLA's best quarterback during the spring and he continued to compete during summer camp to be the team's starting quarterback as a true freshman On August 26, a week after being publicly criticized by head coach Jim Mora, Rosen was named the Bruins' starting quarterback. He beat out Jerry Neuheisel, who had three years of experience in offensive coordinator Noel Mazzone's up-tempo spread offense, which was similar to the offense Rosen had run in high school. The freshman called Neuheisel "selfless" for helping him during the competition. UCLA entered the 2015 season ranked No. 13 by the Associated Press and No. 14 in the Coaches Poll In the season opener, Rosen completed 28 of 35 passes for 351 yards and three touchdowns as UCLA defeated Virginia 34–16 and he became the first true freshman to start a season opener at quarterback for UCLA. He was honored as the Walter Camp Offensive Player of the Week. In a 17–9 win over No. 18 Utah, he broke Drew Olson's decade-old school record (199) for consecutive passes without an interception. The victory kept UCLA in contention for the Pac-12 championship entering their regular-season finale against their cross-town rivals, the USC Trojans. However, they lost 40–21 to the Trojans as Rosen turned the ball over three times on a fumble and two interceptions, ending his streak of passes without an interception at 245. He was named the Pac-12 Freshman Offensive Player of the Year and earned Freshman All-American honors from
USA Today and
Sporting News.