An unidentified military official quoted in the report warned of a strong and crushing response to any violations of the country's airspace by American drone aircraft.
"An advanced RQ170 unmanned American spy plane was shot down by Iran's armed forces. It suffered minor damage and is now in possession of Iran's armed forces," IRNA quoted the official as saying.
No further details were published.
Iran is locked in a dispute with the U.S. and its allies over Tehran's disputed nuclear program, which the West believes is aimed at developing nuclear weapons. Iran denies the accusations, saying its nuclear program is entirely peaceful and that it seeks to generate electricity and produce isotopes to treat medical patients.
Iran said in January that two pilotless spy planes it had shot down over its airspace were operated by the United States and offered to put them on public display.
The Islamic Republic holds frequent military drills, primarily to assert an ability to defend against a potential U.S. or Israeli attack on its nuclear facilities.
Tehran has focused part of its military strategy on producing drones for reconnaissance and attacking purposes.
Iran announced three years ago it had built an unmanned aircraft with a range of more than 600 miles (1,000 kilometers), far enough to reach Israel.
Ahmadinejad unveiled Iran's first domestically built unmanned bomber aircraft in August 2010, calling it an "ambassador of death" to Iran's enemies.