According to Press TV, the protocol was signed by Moscow and Damascus in January 2017, regulating issues related to the Air Forces’ deployment to the Syrian territory.
It was adopted by the Russian State Duma, the Lower House of the Russian Parliament, on July 14, and approved by the Senate five days later.
Vladimir Putin signed it on Thursday, RT cited a Kremlin statement as saying.
Enshrined in the protocol is an option enabling automatic extension of the arrangement for another 25 years.
The protocol also features Damascus’ agreement to provide the Russian air force with free land in its northwestern Latakia Province.
Russia has been using the Khmeimim Air Base there to carry out anti-terror operations since September 2015.
The Russian military has also been offering advisory support to Syria, helping the Arab nation make numerous gains in its battles on terrorists.
In March 2016, Moscow withdrew many of its forces from Syria, with Putin saying the goals of the anti-terrorist mission there had been “generally accomplished.”
Russia, however, said it would keep a military presence at the Port of Tartus and at the Khmeimim Air Base to monitor the situation and the implementation of truce deals.