The World Food Programme aid worker had been detained by the government of South Sudan since April 10. Other aid workers have also been detained since civil war broke out in 2013 in the world's youngest nation.
South Sudan's government has released a United Nations aid worker after detaining him for nearly a month, a top UN official said late on Thursday.
Other aid workers have been detained, and at least 82 killed, since civil war broke out in 2013 in South Sudan, which is increasingly split along ethnic lines.
"We are relieved to learn that Peter Alex, a World Food Programme aid worker detained by the government of South Sudan since April 10, has finally been released and reunited with his family," Nikki Haley, the US ambassador to the UN, said.
"Unfortunately, this is not a unique incident in South Sudan, the most dangerous country in the world today for aid workers," the statement from Haley said.
The United States has accused South Sudanese President Salva Kiir of contributing to the famine and called on all sides to stop fighting.
"The warring parties in South Sudan must stop the ongoing violence," Haley said. "The government of South Sudan must stop obstructing humanitarian assistance and ensure the safety and security of all humanitarian aid workers."
A South Sudanese government spokesman said that Haley's statement was "unfortunate."
"The reality on the ground contradicts the very statement," Deputy Minister of Information Akol Paul Kordit said.
"We as the government are aware of the challenges that are facing us in terms of security, whether insecurity to aid workers or even to our own citizens ... This is something we have committed ourselves to work on to stabilise the country."