"This hunger crisis is on a scale we’ve not seen before and the evidence shows it’s going to get worse," World Food Programme (WFP) regional director Lola Castro said in a statement.
The agency warned that it had only secured $205 million (184 million euros) of the $489 million it requires.
"If we don’t receive the necessary funding, we'll have no choice but to assist fewer of those most in need, and with less," Castro said.
Low growth, rising population, drought and floods have combined to worsen food insecurity in the region, with Eswatini, Lesotho, Madagascar, Malawi, Mozambique, Namibia, Zambia and Zimbabwe the worst-hit.
Nearly half of Zimbabwe's 15 million people live in a state of chronic food insecurity -- some 2.2 million in urban areas and five million in the countryside, according to UN figures.
Twenty percent of the population in drought-stricken Lesotho and about 10 percent of Namibians are food insecure.
In October, Zambia's Red Cross flagged that the drought had left an estimated 2.3 million people facing "severe food insecurity". The country was a longtime regional breadbasket.
The WFP warned that families across the region were already eating less, skipping meals, taking children out of school, selling off precious assets and falling into debt to stave off agricultural losses.
Women and children are the worst affected.
Meanwhile, experts have forecast more months of hot and dry weather in the coming months, auguring another poor harvest.
The situation could escalate further as the dry season may last longer than usual, affecting the annual cereal harvest in April.
The WFP plans to provide lean season assistance to 8.3 million people in areas that are grappling with crisis levels of hunger.