A mortar round hit the compound of the Russian embassy in the Syrian capital Damascus on Sunday, the Syrian Observatory for Human Rights said.
There were no immediate reports of injuries in the attack, which struck inside the compound but did not appear to hit the embassy's main building.
Rebel forces arrayed in parts of the outskirts of the capital regularly fire rockets and mortar rounds into Damascus, including the area around the embassy in the centre of the city.
But the attack is the first time that they have hit the embassy compound, the Observatory said.
Russia is a staunch ally of the regime of Syrian President Bashar al-Assad, providing him with diplomatic backing as well as weapons.
It was a key force behind a deal under which Damascus will hand over its chemical weapons stocks for destruction, which headed off a US plan to take military action against the regime.