No further information was available about the threat, which created a logjam of traffic leaving the Ivy League university in Princeton, N.J.
"Faculty and staff are directed to go home or assemble at evacuation sites as instructed, and students are directed to go into the town of Princeton, where the Nassau Inn, Princeton Public Library and Princeton Arts Council buildings are open to them," the university said in a statement.
Staff were not expected to return the rest of the day. Princeton's public schools did not shut down after the threat but limited its outdoors activities.
A spokesman for Princeton University police could not immediately be reached for comment.
The threat came at a sleepy time for the campus, which held graduation ceremonies last week. The school reported that most of its students had left campus for the summer.
The alerts, however, jarred the university, forcing the shutdown of the school museum and radio station.