It happened around 11:45 p.m. Wednesday outside the Ronald Tutor Campus Center, where about 100 people had gathered for a party hosted by a student organization.
USC Department of Public Safety Capt. David Carlisle says an argument between two men not tied to the school led to one of them pulling out a gun and shooting another person.
That victim was critically wounded. Three bystanders were taken to the hospital with less serious injuries.
The Los Angeles Times reports USC sent a text alert telling students to stay inside. Hours later, another text said the threat was over and classes would go on as scheduled.