IRGC forces have been assisting in the fight against ISIS in both Iraq and Syria, but the missile strike represents a significant escalation in Iran's involvement.
Several Iranian news agencies reported on Sunday evening that Iran has fired several missiles at the bases of militant groups in eastern Syria.
The launch of surface-to-surface medium range missiles into Syria's Deir Ezzor province comes as ISIL militants fleeing a US-led coalition onslaught increasingly try to fortify their positions there.
Iranian television showed footage of the missiles being launched into the night sky.
Five Islamic State-linked attackers stormed Iran's parliament and a shrine to revolutionary leader Ayatollah Ruhollah Khomeini on June 7, killing at least 17 people and wounding more than 50.
Islamic State issued a statement claiming responsibility for the Tehran attack.
The IRGC had vowed revenge for the attacks and accused Saudi Arabia of supporting ISIS in the operation. Iran has arrested dozens of terrorists since the Tehran attacks.
Sunday's assault marked an extremely rare direct attack from the Islamic Republic amid its support for embattled Syrian president Bashar Assad.
Iran has sent to Syria military advisers as well as thousands of "volunteer" fighters recruited among its own nationals as well as the Shiite communities in neighbouring Afghanistan and Pakistan since Syria's conflict broke out in March 2011.