A monitor said clashes and shelling had halted in the three southern provinces covered by the truce, Daraa, Quneitra, and Sweida, as it went into effect at noon local time.
The ceasefire deal was announced Friday by Russian Foreign Minister Sergei Lavrov, and comes as Syrian government and opposition delegations are due to attend a new round of talks in Geneva from Monday.
"The main fronts in the three provinces between regime forces and opposition factions have seen a cessation of hostilities and shelling since this morning, with the exception of a few scattered shells fired on Daraa city before noon," said Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
The monitor said the ceasefire was holding by early Sunday evening. In Washington, US President Donald Trump wrote on Twitter Sunday that he was pleased fighting had not resumed.
"Syrian ceasefire seems to be holding. Many lives can be saved. Came out of meeting. Good!" he tweeted.
The Syrian regime had announced its own unilateral ceasefire on Monday but fighting had continued on front lines in the three provinces.
The ceasefire deal comes after regime allies Russia and Iran and rebel backer Turkey agreed during talks in May in the Kazakh capital Astana to set up four "de-escalation" zones in Syria.
Implementation of that deal has been delayed as the three sides try to agree who will monitor the zones, one of which is located in southern Syria.
- Rebels concerned -
There has been no official comment from Syria's government on the announcement, and there was no mention of the ceasefire on state television's noon news bulletin.
The Al-Watan newspaper, which is close to the regime, quoted the head of Syria's parliamentary Foreign Relations Committee suggesting the agreement was negotiated in consultation with Damascus.
"No details on the agreement were presented, but the Syrian state has background on it," Boutros Marjana told the newspaper.
"The final word on adding southern Syria to the 'de-escalation' zones belongs to the Syrian state, and there is coordination with Russia on that," he added.
On Friday, before the ceasefire deal was announced, a delegation of rebel factions that attended talks in Astana expressed opposition to any ceasefire for just one part of the country.
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In a statement, the factions said they were concerned about "secret meetings and understandings between Russia, Jordan and America on a deal for the south of Syria, separate from the north."
Such an agreement "would divide Syria, as well as the delegation and the opposition, in two."
- 'Suitable atmosphere for talks' -
The United States has largely stepped back from involvement in the diplomatic process to resolve the Syrian conflict since President Donald Trump took office in January.
Trump said he had discussed the conflict with Russia's President Vladimir Putin in a series of tweets on Sunday, two days after they met on the sidelines of the G20 summit in Hamburg, Germany.
"We negotiated a ceasefire in parts of Syria which will save lives," he wrote on Twitter.
"Now it is time to move forward in working constructively with Russia!"
Earlier on Saturday, Trump's national security adviser HR McMaster had described ceasefire zones as "a priority for the United States".
Washington's involvement in the agreement has been interpreted as a sign it may be cautiously re-engaging with efforts to end the war, which has killed more than 320,000 people since it began in March 2011 with anti-government protests.
Jordan borders the southern area covered by the truce and is a key supporter of the main moderate rebel faction there.
Israel, which has occasionally launched what it describes as retaliatory fire against government positions in Quneitra province, said it would "welcome a genuine ceasefire in Syria".
"But this ceasefire must not enable the establishment of a military presence by Iran and its proxies in Syria in general and in southern Syria in particular," Prime Minister Benjamin Netanyahu said.
A UN official on Saturday said the deal created positive momentum ahead of the Geneva talks resuming.
"It helps create a suitable atmosphere for the talks, and we will see that on Monday," said Ramzi Ezzedine Ramzi, deputy to UN Syria envoy Staffan de Mistura.
Expectations for the seventh round of UN-sponsored talks remain low however, with little prospect of a major breakthrough.