That would leave Israel, Egypt and North Korea as the only countries yet to join the arms treaty which came into force in 1997
South Sudan is joining the Chemical Weapons Convention outlawing the use of toxic arms, meaning only three nations have not signed on to the treaty, a global watchdog has said.
"South Sudan has no reason to sit on the fence," top foreign ministry official Moses Akol Ajawin told the annual meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW).
Juba has almost concluded the process to become the body's "newest and youngest state party", he said, according to a statement from the OPCW.
That would leave Israel, Egypt and North Korea as the only countries yet to join the arms treaty which came into force in 1997.
"The images of victims of chemical weapons make us all the more appreciative of the goals and objectives of OPCW," Akol Ajawin said.
"As such, we, in South Sudan, would unreservedly like to associate ourselves with the noble goals and objectives of this great organisation."
The move was welcomed by OPCW chief Ahmet Uzumcu, who said: "Today, we are one step closer to universal membership."
"I urge the other nations that have yet to join the convention -– Egypt, Israel and North Korea –- to unite with the rest of the world in eliminating all chemical weapons forever."
A total of 192 nations have already signed up to the convention, and more than 96 percent of the planet's declared chemical weapon stockpiles have been destroyed under OPCW verification.
South Sudan is the world's newest nation, having gained independence from Sudan in 2011.
But it has been embroiled in a civil war since December 2013 that erupted when President Salva Kiir accused Riek Machar, his former deputy, of plotting a coup.
The United States on Tuesday threatened to take unspecified measures against South Sudan's government unless it moves to end the conflict and stop harassing UN peacekeepers and aid workers.
The US unsuccessfully pushed last year for an arms embargo on South Sudan and international sanctions on senior officials.
Friday's announcement at the OPCW, based in The Hague, came on the final day of the Nobel Peace Prize winning body's annual meeting which had been dominated by the conflict in Syria.
Syria under President Bashar al-Assad finally joined the OPCW in 2013, admitting under US-Russian pressure to having a toxic arms stockpile, and thus staving off threatened US air strikes.
But Damascus came under pressure at this week's meeting to fill the gaps in its 2013 declaration, after an joint UN-OPCW body in its latest report blamed the Syrian air force for a sarin gas attack on the opposition-held village of Khan Sheikhun in April that left scores dead.