The package was reported at about 11:30 a.m. in a basement mailroom where incoming packages are X-rayed, police Officer Albie Esparza said.
One package was deemed suspicious, and the Federal Protective Service, which provides security at the courthouse, asked the Police Department's bomb squad to respond, Esparza said.
The basement and first floor of the building were evacuated, and people on the other floors were told to shelter in place, he said.
Stevenson Street, an alley that runs behind the building, was closed between Sixth and Seventh streets.
Investigators eventually learned that the object was an electronic device that was being delivered to someone in the building, police Lt. Troy Dangerfield said.
Many who work in the courthouse decided to go home before the device was deemed harmless at about 3:40 p.m., according to police.
Bomb-sniffing dogs went through the building after the commotion to check for any additional suspicious items.