STORY: Eyewitness video posted to social media showed passengers and airport workers gathering outside the terminal shortly after the evacuation order.
The San Francisco police said on Twitter that officers received a bomb threat at the airport and found a suspicious package on investigation.
"Due to ongoing police activity the Int'l Terminal is being evacuated," the airport said in a tweet around midnight.
The airport said it had also suspended its AirTrain and Bay Area Rapid Transit trains service following the bomb threat.
The Associated Press reported that police had taken a man into custody.
San Francisco police did not immediately respond to a request asking for more details.