The warning comes from the country’s government and the UN, and it is the first time in six years that famine has been announced anywhere in the world.
The ongoing civil war in South Sudan and the country’s economic collapse have been blamed for the worsening situation.
The famine, which is in two counties, is “man-made,” said Joyce Luma, head of the World Food Programme in South Sudan.
“There is only so much that humanitarian assistance can achieve in the absence of meaningful peace and security.”
Fighting broke out in the East-African nation in 2013 when President Salva Kiir sacked his deputy, accusing him of plotting a coup against him, and has continued ever since.
Tens of thousands of people have died since war broke out, and the UN has warned South Sudan is at risk of genocide.
UN officials have accused the government of blocking food aid from getting through in some places.
A report by the World Food Programme (WFP), UN children’s agency UNICEF and the Food and Agricultural Organisation (FAO) said the famine was the “worst hunger catastrophe since fighting erupted more than three years ago”.
The report said 5.5 million people, about 50% of South Sudan’s population, will face food insecurity and be at risk of death over the coming months, unless something is done to avert the crisis.
“Our worst fears have been realised,” said Serge Tissot, head of the Food and Agriculture Organisation in South Sudan.
He said civilians were resorting to “whatever plants they can find and fish they can catch”, as the civil war has disrupted the country’s normally fertile land.
The UK’s International Development Secretary Priti Patel said: “This is an urgent and severe crisis, with almost half the population in desperate need.
“Almost five million face the daily threat of going without enough food and water and three million people have been forced from their homes because of ruthless violence and widespread use of rape.
“The UK is ensuring millions of people in South Sudan get urgently needed food, water and medicine, as well as longer term support.”
South Sudan has experienced famine before.
During the war for independence from Sudan in 1998 over 70,000 people died of famine.