Deputy Foreign Minister Sergei Ryabkov said the missiles were a “stabilising factor” that could dissuade “some hotheads” from entering the conflict.
Russia also criticised an EU decision not to renew an arms embargo on Syria.
Meanwhile, the BBC has heard evidence that 200 people were killed in a massacre in western Syria this month.
Opposition activists said they had documented the civilian deaths in al-Bayda and Baniyas after government troops and militias entered the towns.
Ryabkov said the contract for the S-300 missile systems had been signed several years ago.
“We consider these supplies a stabilising factor and believe such steps will deter some hotheads from considering scenarios that would turn the conflict international with the involvement of outside forces,” he was quoted as telling journalists.
Russia’s envoy to Nato, Aleksandr Grushko, said Moscow was acting “fully within the framework of international law”, in delivering the arms.
“We are not doing anything that could change the situation in Syria,” he said. “The arms that we supply are defensive…”
Israeli Defence Minister Moshe Yaalon said the Russian missile systems had not yet left Russia.
“I hope they will not leave, and if, God forbid, they reach Syria, we will know what to do,” he said.
Russia has repeatedly blocked efforts to put more pressure on Syrian President Bashar al-Assad.
Along with the US, it has been leading efforts to organise an international peace conference on Syria next month. (BBC)