USC (29-5) moves on to play Pac-12 foe Washington, which swept Kansas earlier Friday at the Galen Center in Los Angeles.
The Cougars, who advanced with wins over Arizona State and Hawaii last weekend, finished their season at 24-7.
In the intense and well-played first set, BYU served on set point five times but ultimately fell in controversial fashion. Neither team led by more than three points in the set.
BYU's Ciara Parker appeared to have made a key dig with the score tied at 30-30, but officials gave USC the set point.
In the second set, the Trojans scored the last six points to win easily and go ahead 2-0.
The Cougars broke from a 3-3 tie with four quick points in the fourth set, and avoided the sweep 25-21 with a kill by Jessica Jardine off a pass by Camry Godfrey.
That ended USC's 20-set winning streak, which dated back to Nov. 22.
But the Trojans had too much firepower in the fourth set, jumping to a quick 3-0 lead and never trailing. Sophomore Alexa Gray led BYU with 23 kills and Parker had 17 digs. Godfrey had 49 assists.
For USC, Samantha Bricio had 22 kills and Natalie Hagglund recorded 22 digs.