Fabian Moreau intercepted Sam Darnold and returned it 32 yards to USC's nine-yard line before Darnold knocked him out of bounds.
Two plays later, Mike Fafaul completed a seven-yard pass to Jordan Lasley, in front of Adoree' Jackson, for a touchdown.
UCLA leads No. 13 USC, 14-7, with 13:28 to play in the second quarter. The scoring drive covered nine plays in two yards and took 40 seconds.