He said President Vladimir Putin and US President Donald Trump agreed on the truce deal during talks held in a "constructive atmosphere" on the sidelines of the G20 summit in Hamburg, Germany. Lavrov said the ceasefire would take effect from July 9.
Earlier, three US officials, who asked not to be named as they were not authorized to discuss the ceasefire publicly, said the truce was set to take effect Sunday at noon Damascus time. They did not elaborate on the deal and how it would be implemented.
The trio said Moscow and Washington had been discussing a potential deal for some time, but it only reached fruition in the run-up to Trump's first meeting with Putin.
Jordan state news agency Petra reported that the kingdom was also part of the ceasefire.
The agreement is separate from "de-escalation zones" that were to be created under a deal mediated by Russia, Iran and Turkey during the fourth round of peace talks in Astana in May.
Syria has been hit by deadly foreign-backed militancy since March 2011. Russia and the United States have been supporting opposing sides in the conflict in Syria with Moscow backing Syrian President Bashar al-Assad and Washington backing militants fighting to topple the Damascus government.