The Syrian government and rebel groups have agreed to a ceasefire from midnight (22:00 GMT) across the country, 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.
Turkey said all fighting including air strikes would be halted. However, some 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 excluded.
An opposition spokesman confirmed the agreement but a rebel source quoted by Reuters news agency said 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.
The Turkish foreign ministry said it was crucial for all states with influence over the groups in Syria to comply with and to support the ceasefire.
That could be seen as an obvious plea to Washington, among others, the BBC’s Selin Girit reports.