Iranian state television shows images of what it says is an unmanned US drone captured in its airspace, but the US Navy denies losing one.
The Revolutionary Guards said they had brought down a ScanEagle - one of the smaller, less sophisticated drones employed by the Americans.
Rear Admiral Ali Fadavi told the Fars news agency that the drone had conducted several reconnaissance flights over the Gulf in recent days.
But the US Navy said none of its drones was missing in the Middle East.
Other nations in the region, including the United Arab Emirates, also operate ScanEagles - low-cost, long-endurance aircraft with a 10ft (3m) wingspan, Associated Press says.