The Integrated Food Security Phase Classification (IPC) alert also revealed 3.9 million people in total were suffering from "severe" food shortages.
The IPC cannot issue a worse rating for food shortages than "catastrophic".
The number of people experiencing such a chronic lack of food in South Sudan has risen by 80% since this time last year.
Christian Aid is distributing fast-growing vegetable seeds, fishing nets, water purification tablets, plastic sheeting for shelter, hygiene kits and jerry cans for storing water and mosquito nets.
Natalia Chan, the senior advocacy officer for South Sudan at Christian Aid, told Premier: "These official figures just confirm our fears that the situation is extremely dire in certain parts of southern Unity State in particular.
"Areas have been classified as "catastrophic", which is the first time that the situation has reached that level in the two years of the conflict.
"Currently the UN Country Appeal is only 55 percent funded for 2015, so funding is one factor, and access is another factor. So we're appealing to parties to the conflict to enable humanitarian assistance as much as possible."
South Sudan has been in the grip of civil war since August 2013, when political violence escalated surrounding rival forces, one backing between current President Salva Kiir and the other former backing Vice President Riek Machar.
Tens of thousands of people have been killed in the conflict so far, with 1.6 million made homeless inside their own country and another 600,000 fleeing to neighbouring countries as refugees.
Listen to Premier's Aaron James speaking to Natalia Chan here: