Former South Sudanese rebel leader Riek Machar has been sworn in as first vice-president, sealing a peace deal aimed at ending six years of civil war.
President Salva Kiir witnessed the moment at a ceremony at the State House in the capital, Juba.
It is hoped that the new unity government will bring an end to the conflict that has killed about 400,000 people and displaced millions.
However, previous deals were widely heralded only to fall apart.
Saturday’s ceremony took place just before the deadline for an agreement expired.
“For the people of South Sudan, I want to assure you that we will work together to end your suffering,” Mr Machar said after taking the oath.
He then embraced and shook hands with President Kiir.
“We must forgive one another and reconcile,” said Mr Kiir. “I also appeal to the people of Dinka and Nuer (rival ethnic groups) to forgive one another.”
Also present at the ceremony was the leader of Sudan, General Abdel Fattah al-Burhan.
Three other vice-presidents were also sworn in including Rebecca Garang, the widow of South Sudan’s founding father, John Garang.
Under the agreement, the current cabinet has been dissolved to make way for more opposition members.
Correspondents say some issues remain unresolved including power-sharing and the integration of rebel fighters, but the two sides have agreed to form a government and address other matters later.
The deal was announced hours after the United Nations (UN) released a damning report accusing both sides of deliberately starving civilians during their struggle for power.
President Kiir has expressed hope that the transitional three-year period will pave the way for refugees and internally displaced people to return to their homes.
In addition to those killed or displaced, many others have been pushed to the brink of starvation and faced untold suffering.
If the deal holds, it could herald a fresh start in the world’s newest country.
South Sudan became an independent state from Sudan in 2011, marking the end of a long-running civil war. But it did not take long for the promise of peace to crumble.
Just two years after independence, the country returned to violent conflict after President Kiir sacked Machar, then the deputy president in December 2013.
President Kiir had accused Mr Machar of plotting a coup to overthrow him, which Mr Machar denied. -BBC
