The UN's World Food Programme said that the number of people needing food aid in south Sudan had quadrupled in a year.
With more than four million needy people, the WFP hopes to ensure that people have enough food to last until their October harvest.
The surge on internal conflict and drought has been blamed, Southern Sudan's agriculture minister Samson Kwaje said.
A two-decade civil war has kept the country from recovering fully.
It remains one of the least developed parts of the world.
Despite the civil war with the north having ended in 2005, some 2,500 people died in conflicts between rival communities in Southern Sudan last year - far more than in Darfur, the UN said.
Since the end of the war, millions of former refugees have returned home to start their lives again from scratch.
Mr Kwaje blamed the hunger on those internal conflicts, as well as incursions by the brutal Lord's Resistance Army, originally based in Uganda but which now marauds across several countries.
The new figure comes from an annual assessment in which some 2,000 people across Southern Sudan were asked about what food they eat and where it comes from.
The WFP's Leo van der Velden said: "This spike in the number of hungry people in Southern Sudan comes just ahead of the rainy season when roads become blocked and communities are cut off from food assistance."
Tension is rising in Sudan before elections due in April - seen as the first national multi-party elections in 24 years.