Sweeney Hall -- which houses the College of Education -- was ordered locked down shortly after 9 a.m. PDT, and students and staff were instructed to shelter in place after a student reported seeing a man holding a gun in the building, the San Jose Mercury News reported.
The rest of the campus remained open while campus police -- aided by San Jose police and Department of Homeland Security personnel -- conducted a room-by-room search of the building.
The lockdown was lifted at 11 a.m. after the search turned up no suspects, police said.
About 300 people were in the building when the lockdown was ordered, the newspaper said.
"We were pleased to find that the faculty, staff and students who were sheltering in place had done an excellent job of barricading themselves in their offices and classrooms," school spokeswoman Pat Lopes Harris said. "It took a while because they would not open their doors, even when the police came."
Lopes said the school "will stay very vigilant in the next 24 to 48 hours looking out for this individual."