According to a tweet from the university's Twitter account, published shortly before 6 p.m., the possible incident was reported near the Visual Arts Building and the west parking structure.
"Do not attempt to leave campus," the tweet said.
All Wednesday evening classes were canceled, a subsequent tweet said.
At 6:30 p.m., a statement on the university's website said an investigation into "shots fired" was ongoing.
Multiple police vehicles were seen on the campus as officers conducted a search.
Chief Jarrod Burguan of the San Bernardino Police Department said the agency was assisting the CSUSB Police Department in the investigation. There were no reports of injuries, Burguan said on Twitter.
The all-clear was issued shortly after 11:30 p.m.
"Everyone not living on campus should immediately leave," the university said in a statement, adding that police would continue to monitor the campus.
Cal State police confirmed that at least one bullet struck the Visual Arts Building.
Classes are scheduled to resume on Thursday.