The Syrian government and rebel groups have agreed a nationwide ceasefire from midnight local time (22:00 GMT), followed by peace talks.
The deal was announced by Russian President Vladimir Putin and confirmed by the Turkish foreign ministry.
Russia and Turkey, which back opposing sides in the bitter conflict, will act as guarantors.
A number of Syrian rebel spokesman have confirmed the deal, but certain jihadist groups are excluded.
The Syrian army said in a statement that so-called Islamic State (IS) and Jabhat Fateh al-Sham (formerly the Nusra Front) “and the groups affiliated to them” were not part of the agreement.
A rebel source quoted by Reuters news agency said that only IS areas were not covered by the truce.
The ceasefire reportedly does cover the rebel-held Eastern Ghouta area near Damascus, which had been a sticking-point in negotiations.
Earlier this month, Moscow and Ankara negotiated a ceasefire in Syria’s second city, Aleppo, that led to tens of thousands of rebel fighters and civilians being evacuated from an enclave besieged by government forces.
Previous ceasefire initiatives this year brokered by the UN, or the US acting with Russia, quickly collapsed.
Mr Putin announced in Moscow that three documents had been signed:
1) An agreement between the Syrian government and the armed opposition on a ceasefire
2) Measures for overseeing the ceasefire
3) An agreement to start peace talks
He described the deal as “fragile” but he praised the agreements as the result of the work of Russia’s defence and foreign ministries with Moscow’s partners in the region.
He added that he agreed with a proposal by the defence ministry to reduce Russia’s military presence in Syria but made it clear Moscow would “continue fighting international terrorism and supporting the Syrian government”.