Bush, who plays for the San Francisco 49ers, attended last Saturday's game at Levi's Stadium to take part in a ceremony that honored the Pac-12 Networks' All-Century team. He was greeted warmly by USC players before the game and by the crowd when he was introduced before kick-off.
Bush won the Heisman Trophy at USC in 2005 but later forfeited the award in the aftermath of an NCAA investigation and sanctions that centered on Bush and his family.
As part of the sanctions, USC was ordered to permanently disassociate from Bush.
A USC spokesman said at the game that Bush was walking in the concourse outside the locker room and "poked his head" in. The spokesman said USC did not orchestrate the locker room visit. Photos were posted on the athletic department's Twitter feed.
"USC was in contact with the NCAA as soon as it learned last week that Reggie Bush would be acknowledged by the Pac-12 on Dec. 5 at Levi's Stadium as a member of the Pac-12 All-Century team," said David Roberts, USC's vice president for athletic compliance. "After the game, USC has followed up with the NCAA regarding Reggie's activities at the game."
The NCAA declined to comment.