Putin justified the delivery of S-300 missiles at a joint press conference during a summit meeting with European Union President Herman Van Rompuy and European Commission President Jose Manuel Barroso, RIA Novosti reported.
While saying Russia did not want to disturb the balance of power in region, Putin said the contract with Syria was signed several years ago.
"It has not yet been realized," he added.
The deliveries were legal, he said, because they were "implemented on the basis of recognized and transparent international contracts."
"They do not violate any international position," Putin added. "And they are implemented exclusively, and in their entirety, within the framework of international law."
The missile systems are capable of engaging 12 targets simultaneously from 125 miles away and at a height of up to nearly 17 miles, he said.