Chancellor William Jenkins called for the evacuation because of a threat phoned into 911 about 10:32 a.m., university spokeswoman Kristine Calongne said. The caller didn't direct the threat to any specific area of the campus.
The university put out a statement on its website announcing the evacuation an hour later, then distributed the information through text messages, emails and social media.
"A bomb threat has been reported on the LSU campus," the statement said. "Please evacuate as calmly and quickly as possible."
There are 30,000 students, professors and university employees located on the Baton Rouge campus, but it was not clear how many were there at the time of the threat.
"Monday ... is a very big class day, so I think the majority of that group was probably on campus at the time," Calongne said.
By mid-afternoon, the LSU campus was largely deserted and roads were closed, though some people and cars were still moving around. Police officers with dogs combed through buildings, including the computer services center.
State police bomb technicians were on the scene, said Louisiana State Police Capt. Doug Cain. He said authorities were talking to their counterparts in Texas, North Dakota and Ohio to see if there were any similarities to threats universities in those states received Friday.
University officials in those states also evacuated their campuses, but police found no explosives.
"It's kind of been an epidemic. This has been the fourth in a week. But it's better to be safe than sorry," said communications disorders graduate student Joseph Vera.
Vera and a fellow graduate student were working in a language clinic with seven children near the edge of campus when they received the text message about the bomb threat. The pair walked the children across the street to an off-campus restaurant and they called the children's parents.
The university sent a follow-up message to students at 1:36 p.m. telling them not to return.
Catherine Lacoste, an 18-year-old freshman and architecture major, said she received notification by text message while working in a studio on a project. She double-checked the information and then evacuated.
"I'm going to go home, take a nap and hopefully campus will be open again when I wake up," Lacoste said.
Kayla Johnson, 18, an English major, heard about the evacuation from a student who received the text message.
"I was in the middle of class and one of the guys in the back of the room raised his hand and said, 'The reason it's so loud outside is because there's been a bomb threat and we have to leave,'" Johnson said.
Students largely seemed to take the evacuation in stride.
"Nobody seems too worried about it," said Shelby Miller, 18, a biology major who was doing homework and eating Chinese food at the student union when she got word of the evacuation.
Miller headed to a nearby coffee shop right off campus to finish her homework.
Calongne said she doesn't know of any other time the entire flagship university campus was evacuated.
"I've been at LSU since 1990 - if you count my student years - and I don't ever recall us having an evacuation of the whole campus," she said.