Word of the ceasefire emerged as US President Donald Trump met Russian President Vladimir Putin at the G20 summit.
The deal marks a new level of involvement for the US in trying to resolve Syria’s civil war.
A separate deal to create “de-escalation zones” was brokered by Russia, Turkey and Iran, but not the US.
Follow-up talks this week in Kazakhstan to finalise a ceasefire in those zones failed to reach a deal.
The US and Russia have been backing opposing sides in Syria’s war.
Jordan and Israel also are part of the agreement, one official said.
The two US allies both share a border with the southern part of Syria and have been concerned about violence from the civil war spilling over the frontier.
Moscow has been supporting Syrian President Bashar Assad and Washington is backing rebels who have been fighting Assad. Both the US and Russia oppose the Islamic State group in Syria.
Previous ceasefires in Syria have collapsed or failed to reduce violence for long, and it is unclear whether this deal will be any better.
Earlier in the week, Syria’s military said it was halting combat operations in the south of Syria for four days, in advance of a new round of Russia-sponsored talks in Astana. That move covered the southern provinces of Daraa, Quneitra and Sweida.
Syria’s government briefly extended that unilateral ceasefire, which is now set to expire on Saturday – a day before the US-Russian deal would take effect.
The new agreement will be open-ended, one US official said, describing it as part of broader discussions with Russia on trying to lower violence in the war-ravaged country.
Officials said the US and Russia were still working out the details as Mr Trump and Mr Putin concluded their more than two-hour meeting on Friday.
The US has been wary of letting Iran gain influence in Syria – a concern shared by Israel and Jordan, neither of which wants Iranian-aligned troops massing near their territories.
A US-brokered deal could help the Trump administration retain more of a say over who fills the power vacuum left behind as Islamic State is routed from additional territory in Syria.
Though US and Russian officials had been discussing a potential deal for some time, it did not reach fruition until the run-up to Mr Trump’s meeting with Mr Putin on the sidelines of the G20 summit, officials said.