"The information on the delivery of S-300 air-defence systems to Iran, which has appeared in certain media outlets, does not correspond to reality," Russia's military-technical cooperation agency said in a statement.
Any arms sales to Iran would be carried out "abiding by all international obligations," the agency added.
There have been contradictory reports about whether Russia was to supply S-300s which Iran could use to defend against an airstrike on its nuclear facilities.
Russia's state-owned arms export firm, Rosoboronexport, said in a statement that it was delivering "defensive" anti-aircraft systems to Iran while not commenting on reports it was sending S-300s.
"At present, only systems of a defensive nature are being delivered to Iran, including means of air defence," Rosoboronexport said. "Previously Tor-M1 systems were delivered to Iran."
Tor-M1 air-defence systems, which Iran has acquired, have a shorter range than the sophisticated S-300s.
However the Interfax news agency quoted an unnamed military source as saying the Russian defence ministry was preparing to hand the S-300s over to Rosoboronexport for delivery to Iran.
"The delivery of the S-300s to Iran is expected to be carried out from Defence Ministry storage sites. Currently the S-300s are being prepared for transfer to Rosoboronexport and then their shipment to the customer," he said.
The source said Russia was planning to deliver S-300PMU1 systems, which have a longer range than older versions of the S-300.
"This is a purely defensive weapon, intended to protect especially important sites from aerial attack," he was quoted as saying.
Last week RIA-Novosti news agency reported that Russia was selling S-300s to Iran, citing anonymous sources. Moscow had previously denied such a deal.
Also the deputy head of the Iranian parliament's foreign affairs commission, Esmaeel Kossari, said last week that Russia was in the process of delivering the S-300 systems to Iran.
The reports come amid persistent tension between Iran and the West over Tehran's disputed nuclear programme, which the West fears could be used to develop nuclear weapons.
The United States and its ally Israel have never ruled out an aerial attack against Iran to thwart its nuclear ambitions.
