The deal was reached during the two leaders' meeting in Hamburg on Friday on the sidelines of the G-20 summit of industrialised and developing nations.
In comments carried by Russia's state-run TASS news agency, Russian Foreign Minister Sergei Lavrov said the truce would come into effect at midday Damascus time (0900 GMT) on Sunday.
"The United States has made a commitment that all the groups present there will observe the ceasefire," Lavrov was quoted as saying.
It was not immediately clear which areas of south-western Syria would be covered by the upcoming ceasefire but earlier talks between the US and Russia about a "de-escalation zone" covered Deraa province, on the border with Jordan, and Quneitra, which borders the Israeli-occupied Golan Heights.
US Secretary of State Rex Tillerson said that Jordan is also participating in the effort.
"I think this is our first indication of the US and Russia being able to work together in Syria, and as a result of that we had a very lengthy discussion regarding other areas in Syria that we can continue to work together," Tillerson said.
'Resident feel unsafe'
Previous ceasefires in Syria have collapsed or failed to reduce violence for long, and it was unclear whether this deal would be any better.
Al Jazeera's Natasha Ghoneim, reporting from Gaziantep, said despite such announcements of ceasefires, fighting continues and residents feel unsafe.
"This week there were peace talks in Astana and ahead of those talks, a ceasefire was announced, but to the west of the country, rebels are in a fierce battle with the Syrian forces who are trying to regain control and along the southern border there is sporadic fighting going on, the Syrian opposition is in control for the most part along the border with Jordan, but clearly fighting continues despite these ceasefires," she said.
"We have spoken to residents in the southwest portion of Syria and they say that despite a ceasefire, they don't feel safe and humanitarian aids don't reach them."
The Syrian conflict has killed nearly half a million people, according to the Britain-based Syrian Observatory for Human Rights, and forced millions of others to flee the country.
Russia is one of Syrian President Bashar al-Assad's most important backers and the survival of his regime is critical to maintaining Russian interests there. The US has argued that al-Assad's regime must be toppled and backs "moderate" rebels fighting Syria government forces.