The agreement calls for a nationwide ceasefire to begin on 12 September – the start of the Muslim Eid al Adha holiday.
If the cessation of hostilities holds for seven days, it will be followed by an unlikely military partnership between the US and Russia to target Islamic State and al Qaeda.
The Syrian government has told Moscow it is prepared to comply with the deal.
Opposition forces have also said they will fulfill the peace plan’s requirements if the Syrian government demonstrates it is serious about ending the bloodshed.
Truces agreed in the past have collapsed after both sides failed to hold up their ends of the bargain.
US Secretary of State John Kerry has heralded the agreement, brokered in Geneva, as a potential “turning point” in the long-running civil war which has killed as many as 500,000 people and displaced millions of others.
However, he warned the plan could be described as nothing more than an opportunity until it is successfully enforced.
Russian Foreign Minister Sergei Lavrov, who was also involved in the negotiations, said the deal would allow efficient co-operation in the struggle against terrorism, and expand humanitarian access to Syria’s worst-hit towns and cities.
Mr Kerry and Mr Lavrov have met four times since 26 August – and Moscow has been pressuring the Assad regime to suspend offensive operations against civilian areas and Syria’s armed opposition.
Meanwhile, Washington was tasked with persuading “moderate” rebels to break away from the Nusra Front and other extremist groups.
Presidents Barack Obama and Vladimir Putin also recently held lengthy face-to-face talks in China.
The announcement of the deal in Geneva was delayed for several hours by internal US discussions.
As tired journalists awaited news, Mr Lavrov took it upon himself to deliver boxes of pizza “from the US delegation”, followed by two bottles of vodka “from the Russian delegation”.
The agreement came as dramatic footage showed bombed-out buildings and rubble on the streets of Aleppo in the aftermath of fierce fighting.
News Source SkyNews