San Francisco police Officer Wilson Ng said the package was not hazardous, but said he had no other details.
A representative of the U.S. General Services Administration, which manages the building, said an "all clear" announcement was made at 4:08 p.m.
The package arrived in the basement mailroom of the building, according to San Francisco police Sgt. David Dorantes, who said Federal Protective Service officers called city police at about 2:45 p.m. after being unable able to ascertain the contents of the package.
During the probe, workers in the 20-story building were instructed to shelter in place. They were allowed to leave from an exit on the west side of the building but no one was permitted to enter.
Police temporarily blocked off Larkin and Polk streets between Golden Gate Avenue and Turk Street on the east and west sides of the building, but allowed traffic to proceed normally on Golden Gate and Turk.
A bomb squad truck and at least six other police vehicles were parked in the closed section of Larkin Street during the investigation.
The GSA's regional spokeswoman, Tracy Madison, referred questions to the Federal Protective Service's spokesman in Washington, D.C., who was not immediately available for comment.