South Sudan MPs on Thursday voted unanimously to approve the extension of President Salva Kiir's term for three more years.
"The transitional constitution amendment bill number five for the year 2018 is hereby passed by the national legislature," said Speaker Anthony Lino Makana after legislators voted to pass the bill which must now be signed into law by Kiir.
The lawmakers said the extension was to avoid power vacuum.
President Kiir terms ends next month, according to a 2015 peace agreement signed in Addis Ababa.
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.
The US recently, warned that it will not recognise the extension of President Kiir's tenure.
South Sudan attained independence in 2011 from Sudan but plunged into civil war after two years.
Since then tens of thousands have been killed and millions forced from their homes, triggering a regional refugee crisis and large-scale hunger.
The parties to the war are currently attending negotiations in Khartoum.
They are expected to sign the final document by the end of this week, according to Information Minster Michael Makuei.
—Additional reporting by AFP