The deal was reached during the two leaders' meeting in Hamburg on Friday on the sidelines of the G20 summit of industrialised and developing nations.
"Russian, American and Jordanian experts ... agreed on a memorandum of understanding to create a de-escalation zone" in the regions of Deraa, Quneitra and Suweida, Russian Foreign Minister Sergei Lavrov said on Friday.
"There will be a ceasefire in this zone from midday Damascus time [9:00 GMT] on July 9," he added.
US Secretary of State Rex Tillerson confirmed the agreement, saying that it showed that the US and Russia were able to work together in Syria and that they would continue to do so.
"We had a very lengthy discussion regarding other areas in Syria that we can continue to work together on to de-escalate the areas and the violence, once we defeat ISIS," he said, using an acronym for the Islamic State of Iraq and the Levant (ISIL) group.
'Resident feel unsafe'
Previous ceasefires in Syria have collapsed or failed to reduce violence for long, and it was unclear whether this deal would fare any better.
Al Jazeera's Natasha Ghoneim, reporting from Gaziantep, Turkey, said that 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 in the west of the country, rebels are in a fierce battle with the Syrian forces who are trying to regain control," she said.
"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.
"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 aid doesn't reach them."
The United Nations welcomed the agreement between the US and Russia, saying it would enable upcoming peace talks.
The UN's deputy special envoy to Syria Ramzy Ramzy said the agreement was a "positive development".
He said he hopes other areas in Syria will see similar agreements to reduce violence.
Ramzy is in Damascus ahead of a new round of UN-sponsored peace talks in Geneva on Monday.
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 Assad's regime must be toppled and backs "moderate" rebels fighting Syria government forces.
Source: Al Jazeera and news agencies