27 August 2015 Last updated at 11:06 BST
It means President Kiir will enter into a government of national unity with rebel leader, Riek Machar.
The BBC's Alastair Leithead was at the signing on Wednesday and sent this report.
South Sudan's President Salva Kiir has signed a peace deal in the capital, Juba, ending a 20-month conflict with rebels, despite voicing reservations about the agreement.