The army downed the unmanned aircraft over Khuzestan province before dawn using a Mersad surface-to-air missile, Tasnim news agency reports.
Iran shot down an unidentified drone near the port of Bandar-e-Mahshahr on the Gulf coast on Friday, Iranian media reported, four-and-a-half months after the downing of a US drone nearly triggered air strikes.
The army downed the unmanned aircraft over Khuzestan province before dawn using a Mersad surface-to-air missile, Tasnim news agency said.
But provincial governor Gholamreza Shariati said the army had located the wreckage of the drone that "belong to a foreign country" in a marshy area and opened an inquiry into the incident.
"The downed drone definitely belonged to a foreign country. Its wreckage has been recovered and is being investigated," Shariati was quoted by state-media IRNA.
It later said the drone could have been an Iranian model downed during nighttime exercises to test missile systems.
The accidental crash of an Iranian drone in the same province on October 23 prompted the newspaper reports that an unidentified foreign aircraft had been shot down.
Fars news agency later revealed that the drone was Iranian and had been forced to make an emergency landing.
The US Central Command said in a statement that all Pentagon drones are accounted for.
"Alleged reports of a US drone being shot down are incorrect," US Central Command said in a post on Twitter.
The incidents come amid soaring tensions between Tehran and Washington since US President Donald Trump unilaterally abandoned a landmark 2015 nuclear deal last year and reimposed crippling sanctions.
Trump in June authorised a military strike after Iran shot down a US drone, only to call off the retaliation at the last moment.
Washington insists the aircraft was in international airspace, but Iran says it was over its territorial waters.
The crisis deepened with the September 14 attacks on Saudi energy giant Aramco's Abqaiq processing plant and Khurais oilfield which halved the kingdom's crude output.
Yemen's Iran-backed Houthi rebels claimed responsibility, but the US says the attacks involved cruise missiles from Iran and amounted to "an act of war".
Washington has responded with a military build-up in the Gulf and has launched an operation with its allies to protect navigation in Gulf waters.
Since May, the US has increased its 70,000-strong presence in the Middle East by 14,000 personnel, according to Pentagon figures, most of them deployed to the Gulf region.