The quake, with an epicenter at 36 kilometers (22 miles) southeast of the Iranian city of Abdanan, near the border with Iraq, struck at 02:32 GMT Monday, the USGS reported.
The quake epicenter was recorded at a depth of 10 kilometers (6.2 miles), the USGS said.
There were no immediate reports of damages or casualties.
Iran stands on several seismic fault lines.
In April last year, a 7.8 magnitude earthquake struck southeastern Iran and neighboring Pakistan, killing 40 people.
A massive quake in December 2003 struck the southern city of Bam, killing 26,000 people and destroying its ancient mud-built citadel.