Spokeswoman Kseniya Grebenkina told The Associated Press that the luggage had been seized some time ago, but could not specify when. The Iranian hasn't been detained, she said.
The Federal Customs Service said in a statement that its agents found 18 pieces of metal at Moscow's Sheremetyevo airport after a radiation alert went on. It says the gauges showed that radiation levels were 20 times higher than normal.
It was not immediately clear if the substance could have any use in Iran's controversial nuclear program.
The pieces contained Sodium-22, she said, a radioactive isotope of sodium that could be produced in a particle accelerator. Sodium-22 is a positron-emitting isotope that has medical uses, including in nuclear medicine imaging.
Grebenkina said prosecutors have launched a probe into the incident.
Sergei Novikov, spokesman for the Rosatom nuclear agency, told the AP that the pieces are highly unlikely to have come from Rosatom and said the isotope is produced by particle accelerators, not by nuclear reactors.
In Russia, universities, research institutes and big medical centers have the technology to produce it, he said.
"There is an extremely slim chance that it could have come from Rosatom," he said.
Novikov said that Rosatom has never sold Sodium-22 to Iran, but it has supplied it with other types of medical isotopes.
Grebenkina of the Customs Service insisted that the material seized is not highly radioactive.
The U.S. House of Representatives on Wednesday endorsed harsher sanctions against Iran aimed at derailing its suspected pursuit of nuclear weapons. Iran denies Western allegations that it is seeking to build nuclear weapons.
Russian officials have insisted that the deal with Iran is in line with international agreements and will oblige Tehran to ship all the spent fuel from the plant back to Russia for reprocessing to avoid a possibility of it being used in a covert weapons program.