Without urgent help, an estimated 4 million people in Malawi, 4 million in Zimbabwe, 1 million in Zambia, 400,000 in Mozambique, 500,000 in Lesotho and 200,000 in Swaziland will not have enough food over the next six months, Oxfam said in a statement on Thursday.
The United Nations and aid groups issued similar warnings about Niger and its neighbours in West Africa, where crops were ravaged by drought and locusts, but the world did not respond until the situation had reached a crisis point, Oxfam said.
"People died as a direct result," said Neil Townsend, the group's humanitarian coordinator for southern Africa.
"Now there is an impending crisis in southern Africa .... If rich countries wait, once again, until TV crews arrive before giving enough money, people in southern Africa will pay the price of their neglect."
The situation in southern Africa has not yet reached famine proportions, UN officials say. But the region is caught in a deepening poverty cycle.
Last month, the UN World Food Programme said that funding shortfalls of $187 million meant that only a fraction of those needing food aid in countries such as Malawi, Mozambique, Zambia and Zimbabwe would receive it.
WFP said that Malawi was one of the hardest hit countries in the region.
"People in the region are used to coping when rains fail, but are increasingly unable to do so because of the HIV/Aids epidemic and other economic factors," Oxfam said.
The situation is considered so serious that UN Secretary-General Kofi Annan last month wrote to 27 heads of state, the European Commission and African Development Bank to ask for urgent funding to "avert a catastrophe".
A number responded, but more funding is needed, Oxfam said.
The group also urged world leaders to create a $1 billion UN emergency reserve fund, so that help is immediately available when countries need it. The proposal is on the agenda for a UN summit starting in New York on 14 September.
"Rich countries spend $1 billion every day on supporting their farmers," Townsend said in the statement.
"If they pledged the same amount every year to a permanent emergency fund at the UN, preventable crises like Niger and southern Africa would not happen."