The latest batch of vaccines delivered to South Sudan through the Covax initiative is the country's biggest yet.
It has received more than 150,000 Johnson & Johnson's single-dose jabs on Tuesday, donated by the US.
They will be administered from the start of October onwards in areas outside of the capital, Juba, says the UN's children agency Unicef, which is leading the efforts.
With a population of 11 million, South Sudan is long way off the World Health Organization's target to have 40% of all African populations vaccinated by the end of 2021.
Jesper Moller, Unicef Deputy Country Representative, told the BBC the Johnson & Johnson doses are easier to use as part of the vaccination programme because they only require a single dose.
Earlier this month South Sudan received 59,520 doses of the Oxford-AstraZeneca vaccine from Covax. The country had run out of supplies in July having immunized more than 50,000 people.
The US has pledged more doses of Johnson & Johnson for South Sudan, and an announcement of the exact numbers is expected next week.