"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 leaves just Israel, Egypt and North Korea as the only countries yet to join the arms treaty which came into force in 1997.
"As such, we, in South Sudan, would unreservedly like to associate ourselves with the noble goals and objectives of this great organisation."
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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 per cent of the planet's declared chemical weapon stockpiles have been destroyed under OPCW verification.
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