Three consecutive rainy seasons have failed as the region has recorded its driest conditions since 1981, the UN agency said, AFP reported.
The drought has destroyed crops and inflicted "abnormally" high livestock deaths, forcing rural families who rely on herding and farming to abandon their homes.
Water and grazing land is in short supply and forecasts of below-average rainfall in coming months only threaten more misery, said Mr. Michael Dunford, the WFP's regional director in East Africa.
"Harvests are ruined, livestock are dying, and hunger is growing as recurrent droughts affect the Horn of Africa," he said in a statement.
"The situation requires immediate humanitarian action" to avoid a repeat of a crisis like that of Somalia in 2011, when 250,000 died of hunger during a prolonged drought.
Food aid is being distributed across an arid swathe of Kenya, Ethiopia and Somalia where malnutrition rates are high and some 13 million people are at risk of severe hunger in the first quarter of this year.
Some 5.7 million needed food assistance in southern and southeastern Ethiopia, including half a million malnourished children and mothers.
In Somalia, the number of people classified as seriously hungry is expected to rise from 3.5 million to 4.6 million by May unless urgent interventions are taken.
Another 2.8 million people need assistance in south-eastern and northern Kenya, where a drought emergency was declared in September.
The WFP said US$327 million (S$439.92 million) was required to respond to immediate needs over the next six months and support pastoral communities to become more resilient against recurring climate shocks.
In 2011, failed rains led to the driest year since 1951 in arid regions of Kenya, Somalia, Ethiopia, Djibouti and Uganda.
Experts say extreme weather events are happening with increased frequency and intensity due to climate change, with Africa - which contributes the least to global warming - bearing the brunt.