The law will allow Kiir to remain in power until 2021.
Parliamentarians in South Sudan voted Thursday to extend the mandate of President Salva Kiir's government for an extra three years, in a move likely to deal a blow to peace efforts.
"The transitional constitution amendment bill number five for the year 2018 is hereby passed by the national legislature," said Speaker Anthony Lino Makana after lawmakers unanimously voted to pass the bill which must now be signed into law by Kiir.
The law will allow Kiir to remain in power until 2021 even as he is engaged in the latest round of regional peace negotiations with his former deputy turned rebel leader Riek Machar aimed at ending the four-year-old civil war.
Kiir and Machar went to war in December 2013.
Since then tens of thousands have been killed and millions forced from their homes, triggering a regional refugee crisis and large-scale hunger.
Successive rounds of talks, mediated by a regional trade bloc called the Intergovernmental Authority on Development (IGAD), have so far failed to bring peace or even a lasting ceasefire.
The latest talks have seen another truce agreed while proposals, including power-sharing, with Machar returned to his former post as vice president, are still under discussion.