President Salva Kiir and rebel leader Riek Machar, a former vice president, met in the Ethiopian capital Addis Ababa on Friday, shook hands and prayed together, and agreed to order a halt to fighting within 24 hours - or by Saturday evening.
The agreement came after massive international pressure on both sides to stop a conflict marked by widespread human rights abuses, a major humanitarian crisis and fears the world's youngest nation was on the brink of a genocide and Africa's worst famine since the 1980's.
"The situation is calm. We have not received any attacks since this morning," South Sudan's defence minister, Kuol Manyang, said late on Saturday.
Several independent aid agencies also confirmed that major frontlines around key towns were quiet throughout the day. Kiir and Machar had agreed to a ceasefire in January, but that deal quickly fell apart.
The United Nations food agency meanwhile is warning that there is only a "small window of opportunity" to avert famine, and appealing for relief agencies - who have been subjected to armed attacks and #####ng to be allowed unfettered access.
In their deal, Kiir and Machar agreed to form a transitional government and hold fresh elections, and "open humanitarian corridors ... and to co-operate with the UN" to ensure aid is delivered.
The peace deal, which followed intense lobbying from world leaders and Washington slapping sanctions on senior military commanders, came amid new reports of war crimes committed by both sides and fears that a wave of ethnic killings could result in genocide.
The war has claimed thousands of lives, with more than 1.2 million people forced to flee their homes.
US Secretary of State John Kerry, who was in South Sudan earlier this month to push for peace, said the agreement "could mark a breakthrough".