The agreement allowed Russia to establish its Hmeimim airbase to launch operations in support of ally President Al-Assad's forces
The agreement -- signed between Moscow and Damascus in August 2015 -- allowed Russia to establish its Hmeimim airbase to launch operations in support of ally President Bashar al-Assad's forces.
Putin's official signing off on the pact, that lasts for an "indefinite" period, is a legal move that many believe will now pave the way for Moscow to make the base permanent.
The ratification of the deal, which follows votes by both Russia's parliament and senate, comes as tensions with the West spike over Russia's bombing of rebel-held, besieged eastern Aleppo.
Putin in August asked lawmakers to ratify the deal, which grants Russian forces immunity from prosecution in Syria.
The latest assault by government forces in Aleppo with the support of Russian airpower has sparked Western accusations of potential war crimes.
As relations have slumped, the Kremlin has bolstered its forces in Syria and on Monday announced they will also turn their Soviet-era naval facility in the country into a permanent base.
Moscow is currently believed to have some 4,000 personnel stationed at Hmeimim, deep in government-held territory, along with several dozen warplanes.