The quake’s epicentre is 89km (55 miles) from Bushehr, the USGS said.
People on social media reported feeling the quake across the Gulf in Dubai and Abu Dhabi. There is no word yet on the extent of any damage.
Iran’s nuclear programme is the subject of international concern amid fears it is trying to build nuclear weapons.
Tehran has always insisted its work at Bushehr and other sites is for civilian purposes only.
The USGS said the quake struck at 11:52 GMT at a depth of 10km (6.2 miles).
Iran’s seismological centre in Bushehr province, linked to Tehran University, registered a 6.1 magnitude quake close to the town of Kaki, some 60 miles south of Bushehr.