The U.S. Geological Survey said the magnitude 5.8 quake struck at 2:17 a.m. (2247 GMT) 83 km (52 miles) southwest of Ardabil.
Iran's IRNA news agency said the tremor was relatively strong and caused many people to run out of their homes in panic in the middle of the night.
Iranian media gave no further details about the casualties, but Iranian state TV said the quake was felt in several towns and cities. The European-Mediterranean Seismological Centre (EMSC) said the quake was felt by some 20 million people.
The quake had a shallow depth of 10 km (6.2 miles), which would have amplified the shaking.