Rosen was 35 of 59 for 491 yards and four touchdowns, and Jalen Starks and Soso Jamabo had touchdown runs for the Bruins in the opener for both teams. They overcame a deficit of more than 20 points for the first time since the 2005 Sun Bowl against Northwestern.
Texas A&M quarterback Kellen Mond was stopped short of the first-down marker on a scramble with 20 seconds left to close out the biggest FBS comeback since 2006.
UCLA scored on five straight possessions after trailing 44-10 with 4:08 to play in the third quarter. Rosen threw touchdown passes of 9 and 42 yards to Darren Andrews before finding Theo Howard for a 16-yard score on a broken play with 3:08 remaining.
Trayveon Williams rushed for 203 yards and two touchdowns, and Keith Ford added 114 yards rushing and three touchdowns for Texas A&M. The Aggies lost for only the second time in six openers under coach Kevin Sumlin.
Rosen spent much of the game under constant pressure from Texas A&M for the second straight season until mounting the stunning comeback. After being sacked five times last season in a 31-24 overtime loss at College Station, Rosen was dropped three times and lost two fumbles in his return after missing the final six games last year with a shoulder injury.
Texas A&M had largely dominated the line of scrimmage on both sides of the ball before giving up touchdowns, rushing for 382 yards and limiting UCLA to 70 yards on the ground. Williams had a 72-yard run where he bounced outside and raced down the UCLA sideline, which set up a 2-yard rushing touchdown by Ford, and the sophomore added a 61-yard scoring run late in the first half.
Redshirt freshman Nick Starkel got the start at quarterback for Texas A&M, and completed his first three throws during an 11-play, 75-yard drive that ended with a 5-yard touchdown run by Ford.
The touted freshman Mond entered the game on the sixth possession for Texas A&M and played the entire second half after Starkel suffered an apparent left foot injury. Starkel returned to the sideline on crutches with his foot in a walking boot.
Mond was 3 of 17 for 27 yards and rushed for 55 yards on 15 carries, and Starkel was 6 of 13 for 62 yards.
Texas A&M: The Aggies return home to host Nicholls State on Saturday