Long Beach Police and campus police immediately responded to the scene.
People were advised people to evacuate the area if able, or to seek shelter if unable to evacuate.
Officials said the incident was not part of a morning drill earlier on campus.
Cal State Long Beach's Twitter page: "An ARMED SUBJECT is in the area of Palo Verde Ave and parking lot 9. Evacuate the area if able, seek shelter if you are unable to evacuate."
Minutes later, CSULB tweeted: "The situation on campus is all clear. There is no threat on campus. Classes will be held as scheduled, and business to resume as usual."
Cal State officials said the report of a gun to university police was a false alarm. Students reportedly mistook a cellphone for a weapon, according to the school.