The object was spotted in a mailbox near Beach and Mason streets shortly after 5 a.m., police Sgt. Michael Andraychak said.
A bomb squad was called in, streets in the immediate area were shut down, and about 30 rooms at a hotel near the mailbox were evacuated, he said.
An employee at the hotel, the Radisson Fisherman's Wharf, said the front desk staff made sure the affected guests were taken care of during the evacuation.
The bomb squad investigated the object, using a remote-controlled robot at one point, and eventually determined that it was harmless.
The streets reopened at about 7:45 a.m.
