The 6-foot-3, 220-pound native of Newport Beach beat out third-year sophomore Aaron Corp and junior Mitch Mustain, a transfer from Arkansas, to become the first true freshman to start at quarterback for the Trojans.
Corp was sidelined two weeks by a leg fracture earlier this month.
"It's clear to us as a staff that Matt Barkley is ready to be the starting quarterback at USC," Carroll said on the school's Web site. "He has exceeded all our expectation (and) has all the physical ability. At this point, he's ready to be the guy for us. The competition will continue at this position, but at this point, Barkley is our starting quarterback."
USC plays its season opener Sept. 5 against San Jose State.